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/AppShell.tsx

@@ -100,14 +100,18 @@ export function AppShell() {
             <StatusPill status={status} />
             {user ? (
               <>
-                <div className="flex items-center gap-2">
+                <Link
+                  to="/account"
+                  title="Account settings"
+                  className="flex items-center gap-2 transition hover:opacity-80"
+                >
                   <span className="flex h-7 w-7 items-center justify-center rounded-full bg-ctp-surface1 font-mono text-xs font-semibold text-ctp-lavender">
                     {user.username.slice(0, 1).toUpperCase()}
                   </span>
                   <span className="hidden text-sm text-ctp-subtext1 sm:block">
                     {user.username}
                   </span>
-                </div>
+                </Link>
                 <button
                   type="button"
                   onClick={() => void 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})`,