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/ui/progress.tsx

@@ -0,0 +1,31 @@
+"use client"
+
+import * as React from "react"
+import { Progress as ProgressPrimitive } from "radix-ui"
+
+import { cn } from "@/lib/utils"
+
+function Progress({
+  className,
+  value,
+  ...props
+}: React.ComponentProps<typeof ProgressPrimitive.Root>) {
+  return (
+    <ProgressPrimitive.Root
+      data-slot="progress"
+      className={cn(
+        "relative h-2 w-full overflow-hidden rounded-full bg-primary/20",
+        className
+      )}
+      {...props}
+    >
+      <ProgressPrimitive.Indicator
+        data-slot="progress-indicator"
+        className="h-full w-full flex-1 bg-primary transition-all"
+        style={{ transform: `translateX(-${100 - (value || 0)}%)` }}
+      />
+    </ProgressPrimitive.Root>
+  )
+}
+
+export { Progress }

frontend/src/components/ui/textarea.tsx

@@ -0,0 +1,18 @@
+import * as React from "react"
+
+import { cn } from "@/lib/utils"
+
+function Textarea({ className, ...props }: React.ComponentProps<"textarea">) {
+  return (
+    <textarea
+      data-slot="textarea"
+      className={cn(
+        "flex field-sizing-content min-h-16 w-full rounded-md border border-input bg-transparent px-3 py-2 text-base shadow-xs transition-[color,box-shadow] outline-none placeholder:text-muted-foreground focus-visible:border-ring focus-visible:ring-[3px] focus-visible:ring-ring/50 disabled:cursor-not-allowed disabled:opacity-50 aria-invalid:border-destructive aria-invalid:ring-destructive/20 md:text-sm dark:bg-input/30 dark:aria-invalid:ring-destructive/40",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+export { Textarea }