e621 credentials: user fields, API endpoint, Account screen

Backend:
- User model gains e621_username / e621_api_key / e621_base_url
- GET/PUT /api/auth/e621/ for the owner's credentials; /me exposes only
  the username and a configured flag, never the key

Frontend:
- e621 client core: Basic auth, _client param (browsers cannot set a
  User-Agent), serialized queue throttled to 1 request/second, readable
  error mapping
- Account screen (/account): username, API key with reveal toggle,
  base URL (e621 / e926 / custom), Save + Test connection
- Credentials are fetched from the backend and held in memory only,
  cleared on logout

frontend/src/components/CommandPalette.tsx

@@ -4,6 +4,7 @@ import {
   LogOut,
   Menu,
   Search,
+  Settings,
   Upload,
   type LucideIcon,
 } from "lucide-react";
@@ -76,6 +77,12 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
       },
     ];
     if (user) {
+      list.push({
+        id: "account",
+        label: "Account settings",
+        icon: Settings,
+        run: () => navigate("/account"),
+      });
       list.push({
         id: "logout",
         label: `Log out (${user.username})`,