feat(ui): add shift management UI with list, detail, and sign-up

Implements Task 20: Shift Sign-Up UI - List + Detail + Create Pages

New components:
- useShifts hook: TanStack Query hooks (useShifts, useShift, useCreateShift, useSignUpShift, useCancelSignUp)
- Shift list page: Card-based UI (NOT table) with capacity bars and Past badges
- Shift detail page: Full info with sign-up/cancel buttons, member list
- New shift form: Create shifts with title, description, location, times, capacity
- ShiftCard component: Reusable card with capacity Progress bar

Key features:
- Card-based UI pattern (shadcn Card, NOT Table - more visual for schedules)
- Capacity calculation: (currentSignups / capacity) * 100 for Progress bar
- Past shift detection: new Date(startTime) < new Date() → shows 'Past' badge
- Sign-up button visibility: !isPast && !isFull && !isSignedUp
- Cancel button visibility: !isPast && isSignedUp
- Role-based 'New Shift' button: Manager/Admin only
- TanStack Query with automatic cache invalidation on mutations
- Navigation includes Shifts link in sidebar

New shadcn components:
- Progress: Radix UI progress bar with transform-based fill
- Textarea: Styled textarea for shift descriptions

TDD:
- 3 shift card tests (capacity display, full capacity, past shifts)
- 3 shift detail tests (sign-up button, cancel button, mutation calls)

All tests pass (37/37: 31 existing + 6 new). Build succeeds.

frontend/src/components/shifts/shift-card.tsx

@@ -0,0 +1,50 @@
+import Link from 'next/link';
+import { Card, CardHeader, CardTitle, CardDescription, CardContent } from '@/components/ui/card';
+import { Button } from '@/components/ui/button';
+import { Progress } from '@/components/ui/progress';
+import { Badge } from '@/components/ui/badge';
+import { ShiftListItemDto } from '@/hooks/useShifts';
+
+interface ShiftCardProps {
+  shift: ShiftListItemDto;
+}
+
+export function ShiftCard({ shift }: ShiftCardProps) {
+  const capacityPercentage = (shift.currentSignups / shift.capacity) * 100;
+  const isFull = shift.currentSignups >= shift.capacity;
+  const isPast = new Date(shift.startTime) < new Date();
+
+  return (
+    <Card>
+      <CardHeader>
+        <div className="flex justify-between items-start">
+          <CardTitle>{shift.title}</CardTitle>
+          {isPast && <Badge variant="secondary">Past</Badge>}
+        </div>
+        <CardDescription>
+          {new Date(shift.startTime).toLocaleString()} - {new Date(shift.endTime).toLocaleTimeString()}
+        </CardDescription>
+      </CardHeader>
+      <CardContent>
+        <div className="space-y-3">
+          <div>
+            <div className="flex justify-between text-sm mb-1">
+              <span>Capacity</span>
+              <span>{shift.currentSignups}/{shift.capacity} spots filled</span>
+            </div>
+            <Progress value={capacityPercentage} />
+          </div>
+
+          <div className="flex gap-2">
+            <Link href={`/shifts/${shift.id}`}>
+              <Button variant="outline" size="sm">View Details</Button>
+            </Link>
+            {!isPast && !isFull && (
+              <Button size="sm">Sign Up</Button>
+            )}
+          </div>
+        </div>
+      </CardContent>
+    </Card>
+  );
+}