Self-service avatar picker in Account

Users can now set their own profile picture instead of asking staff:
POST /api/auth/avatar/ accepts a J-ID (or blank to clear) and reuses the
same item resolution as the staff endpoint. The Account page gains a
profile picture card with a searchable, paginated library grid — any
item works (the thumbnail is used), the current avatar is marked, and
the choice is confirmed before saving. Refreshing the signed-in user
updates the shell avatar immediately.

Verified against the dev server: set, clear, unknown J-ID -> 400,
anonymous -> 401.

frontend/src/features/account/AccountPage.tsx

@@ -13,6 +13,8 @@ import { useAuth } from "@/store/auth";
 import { useE621 } from "@/store/e621";
 import { toast } from "@/store/toasts";
 
+import { AvatarCard } from "./AvatarCard";
+
 const BASE_URL_OPTIONS = [
   { value: "https://e621.net", label: "e621.net — main site" },
   { value: "https://e926.net", label: "e926.net — safe mirror" },
@@ -208,6 +210,7 @@ export default function AccountPage() {
           </p>
         ) : null}
       </header>
+      {user ? <AvatarCard /> : null}
       {loading && !credentials ? (
         <div className="flex justify-center py-12">
           <Spinner className="h-6 w-6" />