feat(ui): add task management UI with list, detail, and create pages

Implements Task 19: Task List + Task Detail + Status Transitions UI

New components:
- useTasks hook: TanStack Query hooks (useTasks, useTask, useCreateTask, useUpdateTask)
- Task list page: shadcn Table with status filter, pagination, status badges
- Task detail page: Full task info with valid status transition buttons
- New task form: Create task with title, description, assigneeId, dueDate

Key features:
- Status transitions match backend logic: Open→Assigned→InProgress→Review→Done
- Review status allows back-transition to InProgress (only bidirectional)
- Only valid next states shown as buttons (VALID_TRANSITIONS map)
- Status badge colors: Open=gray, Assigned=blue, InProgress=yellow, Review=red, Done=green
- TanStack Query with automatic cache invalidation on mutations
- Next.js 15+ async params pattern (use() hook)

TDD:
- 3 task list tests (renders rows, status badges, new task button)
- 3 task detail tests (Open→Assigned, InProgress→Review, Review→Done+InProgress)

All tests pass (31/31). Build succeeds.

frontend/src/app/(protected)/tasks/[id]/page.tsx

@@ -0,0 +1,116 @@
+'use client';
+
+import { use } from 'react';
+import Link from 'next/link';
+import { useRouter } from 'next/navigation';
+import { useTask, useUpdateTask } from '@/hooks/useTasks';
+import { Button } from '@/components/ui/button';
+import { Badge } from '@/components/ui/badge';
+import { Card, CardContent, CardDescription, CardHeader, CardTitle } from '@/components/ui/card';
+
+const VALID_TRANSITIONS: Record<string, string[]> = {
+  Open: ['Assigned'],
+  Assigned: ['InProgress'],
+  InProgress: ['Review'],
+  Review: ['Done', 'InProgress'],
+};
+
+const statusColors: Record<string, string> = {
+  Open: 'bg-secondary text-secondary-foreground hover:bg-secondary/80',
+  Assigned: 'bg-primary text-primary-foreground hover:bg-primary/80',
+  InProgress: 'bg-yellow-500 text-black hover:bg-yellow-500/80',
+  Review: 'bg-destructive text-destructive-foreground hover:bg-destructive/80',
+  Done: 'bg-green-500 text-white hover:bg-green-500/80',
+};
+
+export default function TaskDetailPage({ params }: { params: Promise<{ id: string }> }) {
+  const resolvedParams = use(params);
+  const router = useRouter();
+  const { data: task, isLoading, error } = useTask(resolvedParams.id);
+  const { mutate: updateTask, isPending } = useUpdateTask();
+
+  if (isLoading) return <div className="p-8">Loading task...</div>;
+  if (error || !task) return <div className="p-8 text-red-500">Failed to load task.</div>;
+
+  const validTransitions = VALID_TRANSITIONS[task.status] || [];
+
+  const handleTransition = (newStatus: string) => {
+    updateTask({ id: task.id, data: { status: newStatus } });
+  };
+
+  const getTransitionLabel = (status: string, newStatus: string) => {
+    if (status === 'Review' && newStatus === 'InProgress') return 'Back to InProgress';
+    if (newStatus === 'Done') return 'Mark as Done';
+    return `Move to ${newStatus}`;
+  };
+
+  return (
+    <div className="flex-1 space-y-4 p-8 pt-6">
+      <div className="flex items-center justify-between">
+        <div className="space-y-1">
+          <Button variant="ghost" size="sm" asChild className="mb-2 -ml-3">
+            <Link href="/tasks">← Back to Tasks</Link>
+          </Button>
+          <h2 className="text-3xl font-bold tracking-tight">Task Details</h2>
+        </div>
+      </div>
+
+      <Card>
+        <CardHeader>
+          <div className="flex items-start justify-between">
+            <div>
+              <CardTitle className="text-2xl">{task.title}</CardTitle>
+              <CardDescription className="mt-2 text-base">
+                {task.description || 'No description provided.'}
+              </CardDescription>
+            </div>
+            <Badge className={`text-sm px-3 py-1 ${statusColors[task.status] || 'bg-secondary'}`}>
+              {task.status}
+            </Badge>
+          </div>
+        </CardHeader>
+        <CardContent className="space-y-6">
+          <div className="grid grid-cols-2 gap-4">
+            <div>
+              <p className="text-sm font-medium text-muted-foreground">Assignee</p>
+              <p className="mt-1">{task.assigneeId || 'Unassigned'}</p>
+            </div>
+            <div>
+              <p className="text-sm font-medium text-muted-foreground">Created By</p>
+              <p className="mt-1">{task.createdById}</p>
+            </div>
+            <div>
+              <p className="text-sm font-medium text-muted-foreground">Created At</p>
+              <p className="mt-1">{new Date(task.createdAt).toLocaleString()}</p>
+            </div>
+            {task.dueDate && (
+              <div>
+                <p className="text-sm font-medium text-muted-foreground">Due Date</p>
+                <p className="mt-1">{new Date(task.dueDate).toLocaleDateString()}</p>
+              </div>
+            )}
+          </div>
+
+          {validTransitions.length > 0 && (
+            <div className="pt-6 border-t">
+              <h3 className="text-lg font-medium mb-4">Actions</h3>
+              <div className="flex flex-wrap gap-2">
+                {validTransitions.map((nextStatus) => (
+                  <Button
+                    key={nextStatus}
+                    onClick={() => handleTransition(nextStatus)}
+                    disabled={isPending}
+                    variant={nextStatus === 'Done' ? 'default' : 'outline'}
+                    className={nextStatus === 'Done' ? 'bg-green-600 hover:bg-green-700' : ''}
+                  >
+                    {isPending ? 'Updating...' : getTransitionLabel(task.status, nextStatus)}
+                  </Button>
+                ))}
+              </div>
+            </div>
+          )}
+        </CardContent>
+      </Card>
+    </div>
+  );
+}

frontend/src/app/(protected)/tasks/new/page.tsx

@@ -0,0 +1,129 @@
+'use client';
+
+import { useState } from 'react';
+import { useRouter } from 'next/navigation';
+import Link from 'next/link';
+import { useCreateTask } from '@/hooks/useTasks';
+import { Button } from '@/components/ui/button';
+import { Input } from '@/components/ui/input';
+import { Label } from '@/components/ui/label';
+import { Card, CardContent, CardDescription, CardHeader, CardTitle } from '@/components/ui/card';
+
+export default function NewTaskPage() {
+  const router = useRouter();
+  const { mutate: createTask, isPending, error } = useCreateTask();
+  
+  const [formData, setFormData] = useState({
+    title: '',
+    description: '',
+    assigneeId: '',
+    dueDate: '',
+  });
+
+  const handleChange = (e: React.ChangeEvent<HTMLInputElement | HTMLTextAreaElement>) => {
+    setFormData((prev) => ({ ...prev, [e.target.name]: e.target.value }));
+  };
+
+  const handleSubmit = (e: React.FormEvent) => {
+    e.preventDefault();
+    createTask(
+      {
+        title: formData.title,
+        description: formData.description || undefined,
+        assigneeId: formData.assigneeId || undefined,
+        dueDate: formData.dueDate ? new Date(formData.dueDate).toISOString() : undefined,
+      },
+      {
+        onSuccess: (data) => {
+          router.push(`/tasks/${data.id}`);
+        },
+      }
+    );
+  };
+
+  return (
+    <div className="flex-1 space-y-4 p-8 pt-6">
+      <div className="flex items-center justify-between">
+        <div className="space-y-1">
+          <Button variant="ghost" size="sm" asChild className="mb-2 -ml-3">
+            <Link href="/tasks">← Back to Tasks</Link>
+          </Button>
+          <h2 className="text-3xl font-bold tracking-tight">Create New Task</h2>
+        </div>
+      </div>
+
+      <Card className="max-w-2xl">
+        <CardHeader>
+          <CardTitle>Task Details</CardTitle>
+          <CardDescription>Fill in the details for the new task.</CardDescription>
+        </CardHeader>
+        <CardContent>
+          <form onSubmit={handleSubmit} className="space-y-4">
+            <div className="space-y-2">
+              <Label htmlFor="title">Title <span className="text-red-500">*</span></Label>
+              <Input
+                id="title"
+                name="title"
+                required
+                value={formData.title}
+                onChange={handleChange}
+                placeholder="e.g. Repair tennis net"
+              />
+            </div>
+
+            <div className="space-y-2">
+              <Label htmlFor="description">Description</Label>
+              <textarea
+                id="description"
+                name="description"
+                className="flex min-h-[80px] w-full rounded-md border border-input bg-background px-3 py-2 text-sm ring-offset-background placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-50"
+                value={formData.description}
+                onChange={handleChange}
+                placeholder="Provide additional details..."
+              />
+            </div>
+
+            <div className="grid grid-cols-2 gap-4">
+              <div className="space-y-2">
+                <Label htmlFor="assigneeId">Assignee ID</Label>
+                <Input
+                  id="assigneeId"
+                  name="assigneeId"
+                  value={formData.assigneeId}
+                  onChange={handleChange}
+                  placeholder="Optional"
+                />
+              </div>
+
+              <div className="space-y-2">
+                <Label htmlFor="dueDate">Due Date</Label>
+                <Input
+                  id="dueDate"
+                  name="dueDate"
+                  type="date"
+                  value={formData.dueDate}
+                  onChange={handleChange}
+                />
+              </div>
+            </div>
+
+            {error && (
+              <div className="text-sm text-red-500 mt-2">
+                Error creating task: {error instanceof Error ? error.message : 'Unknown error'}
+              </div>
+            )}
+
+            <div className="flex justify-end pt-4">
+              <Button type="button" variant="outline" className="mr-2" asChild>
+                <Link href="/tasks">Cancel</Link>
+              </Button>
+              <Button type="submit" disabled={isPending || !formData.title.trim()}>
+                {isPending ? 'Creating...' : 'Create Task'}
+              </Button>
+            </div>
+          </form>
+        </CardContent>
+      </Card>
+    </div>
+  );
+}

frontend/src/app/(protected)/tasks/page.tsx

@@ -0,0 +1,131 @@
+'use client';
+
+import { useState } from 'react';
+import Link from 'next/link';
+import { useTasks } from '@/hooks/useTasks';
+import { Button } from '@/components/ui/button';
+import { Badge } from '@/components/ui/badge';
+import {
+  Table,
+  TableBody,
+  TableCell,
+  TableHead,
+  TableHeader,
+  TableRow,
+} from '@/components/ui/table';
+
+const statusColors: Record<string, string> = {
+  Open: 'bg-secondary text-secondary-foreground hover:bg-secondary/80',
+  Assigned: 'bg-primary text-primary-foreground hover:bg-primary/80',
+  InProgress: 'bg-yellow-500 text-black hover:bg-yellow-500/80',
+  Review: 'bg-destructive text-destructive-foreground hover:bg-destructive/80',
+  Done: 'bg-green-500 text-white hover:bg-green-500/80',
+};
+
+export default function TaskListPage() {
+  const [page, setPage] = useState(1);
+  const [statusFilter, setStatusFilter] = useState<string>('');
+
+  const { data, isLoading, error } = useTasks({ 
+    page, 
+    status: statusFilter || undefined 
+  });
+
+  if (error) return <div className="p-4 text-red-500">Failed to load tasks.</div>;
+
+  return (
+    <div className="flex-1 space-y-4 p-8 pt-6">
+      <div className="flex items-center justify-between">
+        <h2 className="text-3xl font-bold tracking-tight">Tasks</h2>
+        <Button asChild>
+          <Link href="/tasks/new">New Task</Link>
+        </Button>
+      </div>
+
+      <div className="flex items-center space-x-2">
+        <select
+          className="flex h-10 w-[200px] items-center justify-between rounded-md border border-input bg-background px-3 py-2 text-sm ring-offset-background placeholder:text-muted-foreground focus:outline-none focus:ring-2 focus:ring-ring focus:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-50"
+          value={statusFilter}
+          onChange={(e) => {
+            setStatusFilter(e.target.value);
+            setPage(1);
+          }}
+        >
+          <option value="">All Statuses</option>
+          <option value="Open">Open</option>
+          <option value="Assigned">Assigned</option>
+          <option value="InProgress">In Progress</option>
+          <option value="Review">Review</option>
+          <option value="Done">Done</option>
+        </select>
+      </div>
+
+      <div className="rounded-md border">
+        <Table>
+          <TableHeader>
+            <TableRow>
+              <TableHead>Title</TableHead>
+              <TableHead>Status</TableHead>
+              <TableHead>Assignee</TableHead>
+              <TableHead>Created</TableHead>
+              <TableHead className="text-right">Actions</TableHead>
+            </TableRow>
+          </TableHeader>
+          <TableBody>
+            {isLoading ? (
+              <TableRow>
+                <TableCell colSpan={5} className="text-center">Loading tasks...</TableCell>
+              </TableRow>
+            ) : data?.items.length === 0 ? (
+              <TableRow>
+                <TableCell colSpan={5} className="text-center">No tasks found.</TableCell>
+              </TableRow>
+            ) : (
+              data?.items.map((task) => (
+                <TableRow key={task.id}>
+                  <TableCell className="font-medium">{task.title}</TableCell>
+                  <TableCell>
+                    <Badge className={statusColors[task.status] || 'bg-secondary'}>
+                      {task.status}
+                    </Badge>
+                  </TableCell>
+                  <TableCell>{task.assigneeId || 'Unassigned'}</TableCell>
+                  <TableCell>{new Date(task.createdAt).toLocaleDateString()}</TableCell>
+                  <TableCell className="text-right">
+                    <Button variant="outline" size="sm" asChild>
+                      <Link href={`/tasks/${task.id}`}>View</Link>
+                    </Button>
+                  </TableCell>
+                </TableRow>
+              ))
+            )}
+          </TableBody>
+        </Table>
+      </div>
+
+      {data && data.total > data.pageSize && (
+        <div className="flex items-center justify-end space-x-2 py-4">
+          <Button
+            variant="outline"
+            size="sm"
+            onClick={() => setPage(p => Math.max(1, p - 1))}
+            disabled={page === 1}
+          >
+            Previous
+          </Button>
+          <div className="text-sm text-muted-foreground">
+            Page {page} of {Math.ceil(data.total / data.pageSize)}
+          </div>
+          <Button
+            variant="outline"
+            size="sm"
+            onClick={() => setPage(p => p + 1)}
+            disabled={page >= Math.ceil(data.total / data.pageSize)}
+          >
+            Next
+          </Button>
+        </div>
+      )}
+    </div>
+  );
+}