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/(auth)/layout.tsx

@@ -0,0 +1,11 @@
+export default function AuthLayout({
+  children,
+}: {
+  children: React.ReactNode
+}) {
+  return (
+    <div className="flex min-h-svh items-center justify-center bg-background p-4">
+      <div className="w-full max-w-sm">{children}</div>
+    </div>
+  )
+}