feat(quick-260630-gbh-01): header avatar display with initial fallback

- AuthUser store: add optional hasAvatar? field
- Header: populate hasAvatar from fetchCurrentUser() result
- Header avatar button: shows <img src='/api-proxy/users/me/avatar'> when hasAvatar=true with onError fallback to initial span
- Plain <img> tag used (same-origin /api-proxy rewrite, no next/image remote config needed)

apps/web/src/lib/stores/auth-store.ts

@@ -6,6 +6,7 @@ export interface AuthUser {
   displayName: string | null;
   role: 'SUPER_ADMIN' | 'ADMIN' | 'USER';
   tenantId: string;
+  hasAvatar?: boolean;
 }
 
 interface AuthState {