feat(account): show the account its own last sign-in

Settings → Account names the latest sign-in on any device (only the
admin's user list did, though the 0.88 notes said otherwise), and the role
in the reader's language instead of the raw `user`.

frontend/src/lib/api/account.ts

@@ -10,6 +10,8 @@ export interface Me {
   avatar_url: string | null;
   role: string;
   is_demo: boolean;
+  /** The latest sign-in on any device (ISO), or null when none is recorded. */
+  last_login_at: string | null;
   has_google: boolean;
   /** The address Google answers with, ONLY when it differs from `email` — the backend does that
    * comparison, so a surface can render this whenever it is present. Someone who registered as