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.
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