feat(onboarding): add max-length validation for household name (100 chars)

Fails fast before the API call with a clear German error message.
Tests boundary: 100 chars accepted, 101 rejected.

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

frontend/src/routes/household/setup/+page.server.ts

@@ -18,6 +18,10 @@ export const actions = {
 			return fail(400, { errors: { name: 'Haushaltsname ist erforderlich' }, name: '' });
 		}
 
+		if (name.length > 100) {
+			return fail(400, { errors: { name: 'Haushaltsname darf maximal 100 Zeichen lang sein' }, name });
+		}
+
 		const api = apiClient(fetch);
 		const { data, error } = await api.POST('/v1/households', {
 			body: { name }

frontend/src/routes/household/setup/page.server.test.ts

@@ -123,6 +123,28 @@ describe('household setup — form action', () => {
 		expect(result.data.name).toBe('');
 	});
 
+	it('returns fail(400) when name exceeds 100 characters', async () => {
+		const longName = 'a'.repeat(101);
+		const result = await actions.default(createRequest({ name: longName }));
+
+		expect(result.status).toBe(400);
+		expect(result.data.errors.name).toBeTruthy();
+		expect(mockPost).not.toHaveBeenCalled();
+	});
+
+	it('accepts name at exactly 100 characters', async () => {
+		mockPost.mockResolvedValue(mockSuccess());
+		const maxName = 'a'.repeat(100);
+
+		try {
+			await actions.default(createRequest({ name: maxName }));
+		} catch {
+			// redirect throws
+		}
+
+		expect(mockPost).toHaveBeenCalled();
+	});
+
 	it('returns fail with form error on API failure', async () => {
 		mockPost.mockResolvedValue({
 			data: undefined,