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/CommandPalette.tsx

@@ -1,5 +1,6 @@
 import { useQuery } from "@tanstack/react-query";
 import {
+  Cable,
   Copy,
   ExternalLink,
   FolderOpen,
@@ -25,6 +26,7 @@ import { useNavigate } from "react-router-dom";
 import { TagChip } from "@/components/TagChip";
 import { Spinner } from "@/components/ui";
 import { TagFollowToggle } from "@/features/follows/TagFollowToggle";
+import { hasBackend } from "@/lib/backend";
 import { cn } from "@/lib/cn";
 import {
   effectiveCredentials,
@@ -112,13 +114,18 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
   });
 
   const commands = useMemo<Command[]>(() => {
+    const backend = hasBackend();
     const list: Command[] = [
-      {
-        id: "library",
-        label: "Go to Library",
-        icon: FolderOpen,
-        run: () => navigate("/"),
-      },
+      ...(backend
+        ? [
+            {
+              id: "library",
+              label: "Go to Library",
+              icon: FolderOpen,
+              run: () => navigate("/"),
+            },
+          ]
+        : []),
       {
         id: "online",
         label: "Go to Online browser",
@@ -131,7 +138,7 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
         icon: Layers,
         run: () => navigate("/pools"),
       },
-      ...(user
+      ...(backend && user
         ? [
             {
               id: "followed",
@@ -147,7 +154,7 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
             },
           ]
         : []),
-      ...(user?.can_upload
+      ...(backend && user?.can_upload
         ? [
             {
               id: "upload",
@@ -176,20 +183,30 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
         icon: Menu,
         run: toggleSidebar,
       },
+      ...(backend
+        ? [
+            {
+              id: "search",
+              label: "Focus library search",
+              hint: "/",
+              icon: Search,
+              run: () => {
+                navigate("/");
+                window.requestAnimationFrame(() => {
+                  document.getElementById("library-search")?.focus();
+                });
+              },
+            },
+          ]
+        : []),
       {
-        id: "search",
-        label: "Focus library search",
-        hint: "/",
-        icon: Search,
-        run: () => {
-          navigate("/");
-          window.requestAnimationFrame(() => {
-            document.getElementById("library-search")?.focus();
-          });
-        },
+        id: "setup",
+        label: backend ? "Change backend" : "Set up a backend",
+        icon: Cable,
+        run: () => navigate("/setup"),
       },
     ];
-    if (user) {
+    if (backend && user) {
       list.push({
         id: "account",
         label: "Account settings",
@@ -210,13 +227,20 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
         icon: LogOut,
         run: () => void logout(),
       });
-    } else {
+    } else if (backend) {
       list.push({
         id: "login",
         label: "Log in",
         icon: LogIn,
         run: () => navigate("/login"),
       });
+    } else {
+      list.push({
+        id: "credentials",
+        label: "e621 credentials",
+        icon: Settings,
+        run: () => navigate("/account"),
+      });
     }
     return list;
   }, [navigate, toggleSidebar, user, logout]);
@@ -326,7 +350,7 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
         <span className="ml-auto font-mono text-[11px] text-ctp-overlay0">
           {row.tag.post_count.toLocaleString()} posts
         </span>
-        <TagFollowToggle tag={row.tag.name} />
+        {hasBackend() ? <TagFollowToggle tag={row.tag.name} /> : null}
         <a
           href={`https://e621.net/posts?tags=${encodeURIComponent(row.tag.name)}`}
           target="_blank"