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.test.ts

@@ -101,6 +101,36 @@ describe('auth guard (hooks.server.ts handle)', () => {
 		expect(resolve).toHaveBeenCalledWith(event);
 	});
 
+	it('handles user without household gracefully', async () => {
+		mockGet.mockResolvedValue({
+			data: {
+				data: {
+					id: '456',
+					displayName: 'Neu',
+					householdId: null,
+					householdName: null,
+					householdRole: null,
+					email: 'neu@example.com',
+					systemRole: 'user'
+				}
+			},
+			error: undefined
+		});
+
+		const { event, resolve } = createEvent('/planner', 'valid-session');
+		await handle({ event, resolve });
+		expect(event.locals.benutzer).toEqual({
+			id: '456',
+			name: 'Neu',
+			rolle: 'mitglied'
+		});
+		expect(event.locals.haushalt).toEqual({
+			id: undefined,
+			name: 'Kein Haushalt'
+		});
+		expect(resolve).toHaveBeenCalledWith(event);
+	});
+
 	it('redirects to /login with redirect param when session validation fails', async () => {
 		mockGet.mockResolvedValue({ data: undefined, error: { status: 401 } });