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/hooks.server.ts

@@ -39,11 +39,11 @@ export const handle: Handle = async ({ event, resolve }) => {
 	event.locals.benutzer = {
 		id: user.id!,
 		name: user.displayName!,
-		rolle: user.householdRole as 'planer' | 'mitglied'
+		rolle: (user.householdRole as 'planer' | 'mitglied') ?? 'mitglied'
 	};
 	event.locals.haushalt = {
-		id: user.householdId!,
-		name: user.householdName!
+		id: user.householdId ?? undefined,
+		name: user.householdName ?? 'Kein Haushalt'
 	};
 
 	return resolve(event);