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/setup/SetupPage.tsx

@@ -5,6 +5,7 @@ import { Button, Spinner, inputClass } from "@/components/ui";
 import { cn } from "@/lib/cn";
 import {
   DEFAULT_BACKEND_URL,
+  NO_BACKEND,
   getBackendUrl,
   getStoredBackendUrl,
   normalizeBackendUrl,
@@ -57,7 +58,10 @@ async function testBackend(base: string): Promise<TestResult> {
  */
 export function SetupPage() {
   const stored = getStoredBackendUrl();
-  const [value, setValue] = useState(stored ?? DEFAULT_BACKEND_URL);
+  const localMode = stored === NO_BACKEND;
+  const [value, setValue] = useState(
+    stored === null || stored === NO_BACKEND ? DEFAULT_BACKEND_URL : stored,
+  );
   const [testing, setTesting] = useState(false);
   const [result, setResult] = useState<TestResult>(null);
 
@@ -101,7 +105,9 @@ export function SetupPage() {
 
           <p className="mt-3 text-sm leading-relaxed text-ctp-subtext0">
             Enter the origin this app should call for its API. Leave it blank
-            when the app and the API are served from the same domain.
+            when the app and the API are served from the same domain. Without a
+            backend the app runs in local mode: e621 browsing only, with
+            credentials kept in this browser.
           </p>
 
           <label className="mt-5 flex flex-col gap-1.5">
@@ -154,6 +160,9 @@ export function SetupPage() {
                 Use this server
               </Button>
             ) : null}
+            <Button variant="ghost" onClick={() => save(NO_BACKEND)}>
+              {localMode ? "Keep running without a backend" : "Continue without a backend"}
+            </Button>
           </div>
 
           <p className="mt-4 text-xs leading-relaxed text-ctp-overlay0">