fix(frontend): restore member self-assignment for shifts and tasks

Root Cause:
- Shift: Next.js 16.1.6 incompatible rewrite pattern caused runtime SyntaxError
- Task: Missing self-assignment UI for member role

Fix:
- Updated next.config.ts rewrite pattern from regex to wildcard syntax
- Added "Assign to Me" button to task detail page with useSession integration
- Added test coverage for self-assignment behavior with session mocks

Testing:
- Lint: ✅ PASS (ESLint v9)
- Tests: ✅ 47/47 PASS (Vitest v4.0.18)
- Build: ✅ PASS (Next.js 16.1.6, 12 routes)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

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

@@ -6,6 +6,7 @@ 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';
+import { useSession } from 'next-auth/react';
 
 const VALID_TRANSITIONS: Record<string, string[]> = {
   Open: ['Assigned'],
@@ -26,6 +27,7 @@ export default function TaskDetailPage({ params }: { params: Promise<{ id: strin
   const resolvedParams = use(params);
   const { data: task, isLoading, error } = useTask(resolvedParams.id);
   const { mutate: updateTask, isPending } = useUpdateTask();
+  const { data: session } = useSession();
 
   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>;
@@ -36,6 +38,12 @@ export default function TaskDetailPage({ params }: { params: Promise<{ id: strin
     updateTask({ id: task.id, data: { status: newStatus } });
   };
 
+  const handleAssignToMe = () => {
+    if (session?.user?.id) {
+      updateTask({ id: task.id, data: { assigneeId: session.user.id } });
+    }
+  };
+
   const getTransitionLabel = (status: string, newStatus: string) => {
     if (status === 'Review' && newStatus === 'InProgress') return 'Back to InProgress';
     if (newStatus === 'Done') return 'Mark as Done';
@@ -93,6 +101,15 @@ export default function TaskDetailPage({ params }: { params: Promise<{ id: strin
             <div className="pt-6 border-t">
               <h3 className="text-lg font-medium mb-4">Actions</h3>
               <div className="flex flex-wrap gap-2">
+                {!task.assigneeId && session?.user && (
+                  <Button
+                    onClick={handleAssignToMe}
+                    disabled={isPending}
+                    variant="outline"
+                  >
+                    {isPending ? 'Assigning...' : 'Assign to Me'}
+                  </Button>
+                )}
                 {validTransitions.map((nextStatus) => (
                   <Button
                     key={nextStatus}