test(admin): expand admin/users/[id] page coverage

Adds banner-hidden defaults (success/error), empty groups list,
groups field undefined fallback to [].

4 new tests covering ~6 branches.

Refs #496.

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

frontend/src/routes/admin/users/[id]/page.svelte.test.ts

@@ -84,4 +84,49 @@ describe('admin/users/[id] page', () => {
 
 		await expect.element(page.getByRole('button', { name: /löschen/i })).toBeVisible();
 	});
+
+	it('does not show success banner when form is undefined', async () => {
+		render(AdminUserEditPage, { props: { data: baseData(), form: undefined } });
+
+		const banner = document.querySelector('.bg-green-50');
+		expect(banner).toBeNull();
+	});
+
+	it('does not show error banner when form.error is undefined', async () => {
+		render(AdminUserEditPage, { props: { data: baseData(), form: { success: false } } });
+
+		// The error banner has both border-red-200 AND text-red-700 — the delete button has red-50
+		// background but is a button, not a div. Look for the specific error <div>.
+		const banner = document.querySelector('div.bg-red-50');
+		expect(banner).toBeNull();
+	});
+
+	it('handles a user with empty groups list (selectedGroupIds defaults to [])', async () => {
+		render(AdminUserEditPage, {
+			props: {
+				data: baseData({ editUser: { ...baseEditUser, groups: [] } }),
+				form: undefined
+			}
+		});
+
+		const checkboxes = Array.from(
+			document.querySelectorAll('input[name="groupIds"]')
+		) as HTMLInputElement[];
+		const checked = checkboxes.filter((c) => c.checked);
+		expect(checked.length).toBe(0);
+	});
+
+	it('handles a user with no groups field at all (defaults to [])', async () => {
+		const editUser = { ...baseEditUser } as typeof baseEditUser & { groups?: undefined };
+		delete (editUser as { groups?: unknown }).groups;
+		render(AdminUserEditPage, {
+			props: { data: baseData({ editUser }), form: undefined }
+		});
+
+		const checkboxes = Array.from(
+			document.querySelectorAll('input[name="groupIds"]')
+		) as HTMLInputElement[];
+		const checked = checkboxes.filter((c) => c.checked);
+		expect(checked.length).toBe(0);
+	});
 });