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

Adds color picker hidden for child tags, form-success default
hidden state, mergeSuccess null render-without-throw.

3 new tests covering ~5 branches.

Refs #496.

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

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

@@ -118,4 +118,36 @@ describe('admin/tags/[id] page', () => {
 		const banners = document.querySelectorAll('[role="status"]');
 		expect(banners.length).toBeGreaterThan(0);
 	});
+
+	it('hides the color picker for child tags (parentId set)', async () => {
+		render(AdminTagEditPage, {
+			props: {
+				data: baseData({ tag: baseTag({ parentId: 't-parent' }) }),
+				form: undefined
+			}
+		});
+
+		const colorPicker = document.querySelector('[data-testid="color-picker"]');
+		expect(colorPicker).toBeNull();
+	});
+
+	it('does not show form-success banner when form is undefined', async () => {
+		render(AdminTagEditPage, { props: { data: baseData(), form: undefined } });
+
+		const banners = document.querySelectorAll('.bg-green-50');
+		// Some other green elements may exist, but the form-success specifically
+		// — accept that the banner may not be present
+		const formSuccessBanner = Array.from(banners).filter((b) =>
+			(b.textContent ?? '').match(/gespeichert|aktualisiert|saved/i)
+		);
+		expect(formSuccessBanner.length).toBe(0);
+	});
+
+	it('renders without throwing when mergeSuccess is null', async () => {
+		expect(() =>
+			render(AdminTagEditPage, {
+				props: { data: baseData({ mergeSuccess: null }), form: undefined }
+			})
+		).not.toThrow();
+	});
 });