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/store/auth.ts

@@ -1,6 +1,7 @@
 import { create } from "zustand";
 
 import { api, getToken, setToken } from "@/lib/api";
+import { hasBackend } from "@/lib/backend";
 import { applyPreferences } from "@/lib/preferences";
 import { queryClient } from "@/lib/queryClient";
 import type { User } from "@/lib/types";
@@ -58,6 +59,11 @@ export const useAuth = create<AuthState>((set) => ({
   },
 
   loadUser: async () => {
+    if (!hasBackend()) {
+      // Local mode: no account, and the e621 features work on their own.
+      set({ user: null, ready: true });
+      return;
+    }
     if (!getToken()) {
       set({ user: null, ready: true });
       return;

frontend/src/store/e621.ts

@@ -1,6 +1,7 @@
 import { create } from "zustand";
 
 import { api } from "@/lib/api";
+import { hasBackend } from "@/lib/backend";
 import {
   effectiveCredentials,
   updateBlacklist,
@@ -9,6 +10,43 @@ import {
   type E621User,
 } from "@/lib/e621";
 
+/** Standalone mode keeps credentials in this browser only. */
+const LOCAL_CREDENTIALS_KEY = "j621.e621";
+
+function readLocalCredentials(): E621Credentials | null {
+  try {
+    const raw = window.localStorage.getItem(LOCAL_CREDENTIALS_KEY);
+    if (!raw) return null;
+    const parsed = JSON.parse(raw) as Partial<E621Credentials>;
+    const username = String(parsed.username ?? "");
+    const api_key = String(parsed.api_key ?? "");
+    return {
+      username,
+      api_key,
+      base_url: String(parsed.base_url ?? "https://e621.net"),
+      configured: Boolean(username && api_key),
+    };
+  } catch {
+    return null;
+  }
+}
+
+function writeLocalCredentials(values: {
+  username: string;
+  api_key: string;
+  base_url: string;
+}): E621Credentials {
+  const credentials: E621Credentials = {
+    ...values,
+    configured: Boolean(values.username && values.api_key),
+  };
+  window.localStorage.setItem(
+    LOCAL_CREDENTIALS_KEY,
+    JSON.stringify(credentials),
+  );
+  return credentials;
+}
+
 interface E621State {
   credentials: E621Credentials | null;
   profile: E621User | null;
@@ -35,6 +73,11 @@ export const useE621 = create<E621State>((set, get) => ({
 
   load: async () => {
     set({ loading: true });
+    if (!hasBackend()) {
+      set({ credentials: readLocalCredentials(), ready: true, loading: false });
+      void get().loadBlacklist();
+      return;
+    }
     try {
       const credentials = await api<E621Credentials>("/api/auth/e621/");
       set({ credentials, ready: true, loading: false });
@@ -71,6 +114,12 @@ export const useE621 = create<E621State>((set, get) => ({
   },
 
   save: async (values) => {
+    if (!hasBackend()) {
+      const credentials = writeLocalCredentials(values);
+      set({ credentials });
+      void get().loadBlacklist();
+      return credentials;
+    }
     const credentials = await api<E621Credentials>("/api/auth/e621/", {
       method: "PUT",
       json: values,