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

backend/config/urls.py

@@ -6,6 +6,7 @@ from django.urls import include, path
 urlpatterns = [
     path("admin/", admin.site.urls),
     path("api/auth/", include("apps.accounts.urls")),
+    path("api/", include("apps.core.urls")),
     path("api/", include("apps.library.urls")),
 ]