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/account/AccountPage.tsx

@@ -1,8 +1,10 @@
 import { Eye, EyeOff } from "lucide-react";
 import { useEffect, useState } from "react";
+import { Link, Navigate } from "react-router-dom";
 
 import { Button, Field, Spinner, inputClass } from "@/components/ui";
 import { errorMessage } from "@/lib/api";
+import { hasBackend } from "@/lib/backend";
 import { cn } from "@/lib/cn";
 import {
   e621ErrorMessage,
@@ -175,8 +177,9 @@ function AccountForm({ credentials }: { credentials: E621Credentials | null }) {
         </div>
 
         <p className="text-xs text-ctp-overlay0">
-          Stored on your server and fetched by this app when needed. e621 requests
-          are made directly from your browser, throttled to one per second.
+          {hasBackend()
+            ? "Stored on your server and fetched by this app when needed. e621 requests are made directly from your browser, throttled to one per second."
+            : "Stored in this browser only (local mode). e621 requests are made directly from here, throttled to one per second."}
         </p>
       </div>
     </section>
@@ -188,35 +191,62 @@ export default function AccountPage() {
   const credentials = useE621((state) => state.credentials);
   const loading = useE621((state) => state.loading);
   const load = useE621((state) => state.load);
+  const backend = hasBackend();
 
   useEffect(() => {
     void load();
   }, [load]);
 
+  // Local mode: the e621 credentials live in this browser.
+  if (!backend) {
+    return (
+      <div className="mx-auto flex w-full max-w-2xl flex-col gap-6">
+        <header>
+          <h1 className="text-lg font-semibold">e621 account</h1>
+          <p className="mt-1 text-sm text-ctp-overlay0">
+            No backend is connected, so these credentials are stored in this
+            browser only and sent straight to e621 from here.
+          </p>
+        </header>
+        {loading && !credentials ? (
+          <div className="flex justify-center py-12">
+            <Spinner className="h-6 w-6" />
+          </div>
+        ) : (
+          <AccountForm credentials={credentials} />
+        )}
+        <p className="text-xs text-ctp-overlay0">
+          <Link to="/setup" className="text-ctp-blue hover:underline">
+            Connect a backend
+          </Link>{" "}
+          to unlock the library, follows and account settings.
+        </p>
+      </div>
+    );
+  }
+
+  if (!user) {
+    return <Navigate to="/login" replace />;
+  }
+
   return (
     <div className="mx-auto flex w-full max-w-2xl flex-col gap-6">
       <header>
         <h1 className="text-lg font-semibold">Account</h1>
-        {user ? (
-          <p className="mt-1 flex flex-wrap items-center gap-2 text-sm text-ctp-overlay0">
-            <span>Signed in as {user.username}.</span>
-            <span className="rounded-full border border-ctp-surface1 px-2 py-0.5 font-mono text-[11px] capitalize text-ctp-subtext0">
-              {user.role}
+        <p className="mt-1 flex flex-wrap items-center gap-2 text-sm text-ctp-overlay0">
+          <span>Signed in as {user.username}.</span>
+          <span className="rounded-full border border-ctp-surface1 px-2 py-0.5 font-mono text-[11px] capitalize text-ctp-subtext0">
+            {user.role}
+          </span>
+          {user.is_superuser ? (
+            <span className="rounded-full border border-ctp-red/40 bg-ctp-red/10 px-2 py-0.5 font-mono text-[11px] text-ctp-red">
+              admin
             </span>
-            {user.is_superuser ? (
-              <span className="rounded-full border border-ctp-red/40 bg-ctp-red/10 px-2 py-0.5 font-mono text-[11px] text-ctp-red">
-                admin
-              </span>
-            ) : null}
-          </p>
-        ) : null}
+          ) : null}
+        </p>
       </header>
-      {user ? (
-        <>
-          <AvatarCard />
-          <PreferencesCard />
-        </>
-      ) : null}
+      <AvatarCard />
+      <PreferencesCard />
       {loading && !credentials ? (
         <div className="flex justify-center py-12">
           <Spinner className="h-6 w-6" />