Redesign teams portal and add user autocomplete for member invitations

- Redesign TeamsPage with modern card-based layout including stats bar,
  search functionality, and empty states
- Add UserAutocomplete component with debounced search and keyboard
  navigation for selecting existing users
- Add /api/v1/users/search endpoint for username prefix search
- Update TeamMembersPage to use UserAutocomplete instead of free text input

frontend/src/api.ts

@@ -668,3 +668,17 @@ export async function listTeamProjects(
   });
   return handleResponse<PaginatedResponse<Project>>(response);
 }
+
+// User search (for autocomplete)
+export interface UserSearchResult {
+  id: string;
+  username: string;
+  is_admin: boolean;
+}
+
+export async function searchUsers(query: string, limit: number = 10): Promise<UserSearchResult[]> {
+  const response = await fetch(`${API_BASE}/users/search?q=${encodeURIComponent(query)}&limit=${limit}`, {
+    credentials: 'include',
+  });
+  return handleResponse<UserSearchResult[]>(response);
+}