Shell polish: storage line, status pill, footer strip, sidebar

Backend:
- apps.core with GET /api/status/ (env, git hash, OS, watched folder,
  worker counts) and a TimingMiddleware adding X-Server-Time-Ms
- status endpoint reports its own server-side assembly time

Frontend:
- top bar shows the storage line (watched folder) and a status pill
  with ENV, git hash, host OS, server time (e621 time once it exists)
- fixed 32px footer strip: storage path, active workers, build version
- collapsible filter sidebar: closed by default, Ctrl/Cmd+B toggle,
  overlay drawer with backdrop blur under 1024px
- rating filters, per-page and sidebar state persist in localStorage

frontend/src/lib/types.ts

@@ -35,3 +35,22 @@ export interface Paginated<T> {
   previous: string | null;
   results: T[];
 }
+
+export interface SystemStatus {
+  app: string;
+  env: "dev" | "prod";
+  git_hash: string;
+  version: string;
+  os: {
+    name: string;
+    release: string;
+    icon: "linux" | "apple" | "windows" | "unknown";
+  };
+  watched_folder: string;
+  e621_time_ms: number | null;
+  workers: {
+    active: number;
+    queued: number;
+  };
+  server_time_ms: number;
+}