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/lib/api.ts

@@ -1,5 +1,5 @@
 import type { TempUpload } from "@/lib/types";
-import { getBackendUrl } from "@/lib/backend";
+import { getBackendUrl, hasBackend } from "@/lib/backend";
 
 const TOKEN_KEY = "j621.token";
 
@@ -66,6 +66,11 @@ interface ApiOptions extends Omit<RequestInit, "body"> {
 }
 
 export async function api<T>(path: string, options: ApiOptions = {}): Promise<T> {
+  if (!hasBackend()) {
+    throw new ApiError(0, {
+      detail: "No backend is connected — set one up to use this feature.",
+    });
+  }
   const { json, headers, ...rest } = options;
   const finalHeaders = new Headers(headers);
   let body = options.body;