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/components/StatusFooter.tsx

@@ -1,4 +1,4 @@
-import { HardDrive, Zap } from "lucide-react";
+import { HardDrive, Unplug, Zap } from "lucide-react";
 import { Link } from "react-router-dom";
 
 import { cn } from "@/lib/cn";
@@ -14,13 +14,15 @@ function storageColor(percent: number): string {
 }
 
 export function StatusFooter({
+  backend,
   status,
   staff,
 }: {
+  backend: boolean;
   status?: SystemStatus;
   staff?: boolean;
 }) {
-  const backend = getBackendUrl() || window.location.origin;
+  const backendOrigin = getBackendUrl() || window.location.origin;
   const storage = status?.storage;
 
   const workers = (
@@ -35,6 +37,26 @@ export function StatusFooter({
     </>
   );
 
+  if (!backend) {
+    return (
+      <footer className="fixed inset-x-0 bottom-0 z-30 border-t border-ctp-surface0 bg-ctp-crust">
+        <div className="mx-auto flex h-8 w-full max-w-[1600px] items-center gap-4 px-4 font-mono text-[11px] text-ctp-overlay0">
+          <span className="flex min-w-0 shrink-0 items-center gap-1.5">
+            <Unplug className="h-3 w-3 shrink-0" />
+            Local mode — e621 features only
+          </span>
+          <Link
+            to="/setup"
+            className="mx-auto transition hover:text-ctp-text"
+            title="Connect a backend to unlock the library, uploads and account features"
+          >
+            Setup Backend
+          </Link>
+        </div>
+      </footer>
+    );
+  }
+
   return (
     <footer className="fixed inset-x-0 bottom-0 z-30 border-t border-ctp-surface0 bg-ctp-crust">
       <div className="mx-auto flex h-8 w-full max-w-[1600px] items-center gap-4 px-4 font-mono text-[11px] text-ctp-overlay0">
@@ -66,7 +88,7 @@ export function StatusFooter({
 
         <span
           className="mx-auto hidden min-w-0 items-center gap-1.5 md:flex"
-          title={`Backend API: ${backend}`}
+          title={`Backend API: ${backendOrigin}`}
         >
           {staff ? (
             <Link
@@ -74,10 +96,10 @@ export function StatusFooter({
               className="truncate transition hover:text-ctp-text"
               title="Change backend"
             >
-              {backend}
+              {backendOrigin}
             </Link>
           ) : (
-            <span className="truncate">{backend}</span>
+            <span className="truncate">{backendOrigin}</span>
           )}
         </span>