fix(auth): handle users without household — fallback to 'Kein Haushalt'

Removes non-null assertions on householdId/householdName. Users who
haven't joined a household get a fallback name in the sidebar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/app.d.ts

@@ -12,7 +12,7 @@ declare global {
 				rolle: 'planer' | 'mitglied';
 			};
 			haushalt?: {
-				id: string;
+				id: string | undefined;
 				name: string;
 			};
 		}