feat: frontend MVP — auth, timeline, upload, albums, admin, image viewer

Backend:
- user roles (DB + JWT + first-user-is-admin)
- volume-aware file resolver (multi-volume asset serving)
- directory scanner uses volume URI directly
- date-summary endpoint (capture date from EXIF)
- timeline ordered by capture date
- list endpoints: volumes, plugins, pipelines, library paths
- delete endpoints: volumes, library paths
- configurable upload body limit (MAX_UPLOAD_BYTES)

Frontend:
- auth: login/register, token refresh, role-based admin gate
- timeline: date-grouped grid, infinite scroll, date scrubber
- image viewer: fullscreen zoom/pan/pinch, metadata sidebar
- upload: drag-drop, sequential upload, progress tracking
- albums: create, add/remove photos, asset picker dialog
- admin: storage (import library), jobs (pagination, error details),
  plugins (list + toggle), pipelines, sidecars, duplicates
- multi-select mode with add-to-album action
- TanStack Query for all data fetching

k-photos-frontend/app/(app)/admin/layout.tsx

@@ -0,0 +1,22 @@
+"use client"
+
+import { useAuth } from "@/hooks/use-auth"
+import { useRouter } from "next/navigation"
+import { useEffect } from "react"
+
+export default function AdminLayout({
+  children,
+}: {
+  children: React.ReactNode
+}) {
+  const { isAdmin, isLoading } = useAuth()
+  const router = useRouter()
+
+  useEffect(() => {
+    if (!isLoading && !isAdmin) router.replace("/")
+  }, [isLoading, isAdmin, router])
+
+  if (isLoading || !isAdmin) return null
+
+  return <>{children}</>
+}