Run the SPA without a backend (local mode)

The app can now operate backend-agnostically: a production build still
asks on first start, but /setup also offers 'Continue without a backend'
(stored as the sentinel 'none'), and the shell adapts:

- Local mode shows only the e621-facing pages: Online (search, post view,
  favorites, blacklist editor, direct downloads) and Pools. Library,
  uploads, duplicates, stats, users, follows and similarity are hidden
  from the nav and palette and render a 'backend needed' state when
  reached directly; /detail/<e621 id> still works while /detail/J-x asks
  for a backend.
- e621 credentials are stored in this browser (j621.e621) and the
  Account page becomes a credentials-only screen; the store reads/writes
  locally instead of /api/auth/e621/.
- The header replaces the status pill and login/user area with an e621
  credentials button and a 'Setup Backend' button; the footer shows
  'Local mode — e621 features only' with the same entry point.
- In-library lookups (badges/browse markers) are skipped without a
  backend; 'Download to client' links straight to the e621 file instead
  of the backend proxy; follow buttons and palette follow toggles are
  hidden; api() fails fast with a clear message if something slips
  through.

Mode logic lives in lib/backend.ts (URL / '' same-origin / 'none') with
its matrix verified in Node; tsc, oxlint and the build are clean.

frontend/src/features/detail/DetailPage.tsx

@@ -1,8 +1,10 @@
 import { useParams } from "react-router-dom";
 
+import { BackendNeeded } from "@/components/BackendNeeded";
 import { EmptyState } from "@/components/ui";
 import { LibraryDetail } from "@/features/library/LibraryDetail";
 import { OnlineDetail } from "@/features/online/OnlineDetail";
+import { hasBackend } from "@/lib/backend";
 
 const J_ID_RE = /^J-\d+$/i;
 
@@ -10,6 +12,7 @@ export default function DetailPage() {
   const { itemId = "" } = useParams();
 
   if (J_ID_RE.test(itemId)) {
+    if (!hasBackend()) return <BackendNeeded />;
     const normalized = `J-${itemId.slice(2)}`;
     return <LibraryDetail jId={normalized} />;
   }