fix(documents): tag click no longer navigates to document detail page

Nesting the tag <button> inside the row's <a href="…"> made the browser
treat any click on the button as a click on the anchor, sending the
user to the document detail page even though the tag handler called
goto() with the tag-filter URL. e.stopPropagation() doesn't cancel
the anchor's default navigation.

Refactor to the stretched-link pattern: the row-wide anchor sits as an
overlay (`absolute inset-0 z-0`) and the content wrapper sits above it
(`relative z-10` + `pointer-events-none`). Tag buttons re-enable
pointer events with `pointer-events-auto`, so they're true siblings of
the anchor and receive their own clicks. Empty content areas pass
through to the anchor for whole-row navigation.

The vitest-browser client project doesn't load Tailwind CSS, so the
z-index has no effect there and Playwright's coordinate-based click
hits the anchor instead of the button. Trigger the click directly on
the button DOM element in the unit test (with a comment explaining the
test-env constraint); the actual user-facing behavior is verified via
playwright against the running dev server.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/DocumentRow.svelte.spec.ts

@@ -1,12 +1,16 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page } from 'vitest/browser';
+import { goto } from '$app/navigation';
 import DocumentRow from './DocumentRow.svelte';
 import type { components } from '$lib/generated/api';
 
 vi.mock('$app/navigation', () => ({ goto: vi.fn() }));
 
-afterEach(() => cleanup());
+afterEach(() => {
+	cleanup();
+	vi.mocked(goto).mockClear();
+});
 
 type DocumentSearchItem = components['schemas']['DocumentSearchItem'];
 
@@ -229,7 +233,6 @@ describe('DocumentRow – tags', () => {
 	});
 
 	it('navigates to /documents?tag=… on tag click', async () => {
-		const { goto } = await import('$app/navigation');
 		const item = makeItem({
 			document: {
 				...makeItem().document,
@@ -237,8 +240,15 @@ describe('DocumentRow – tags', () => {
 			}
 		});
 		render(DocumentRow, { item });
-		await page.getByRole('button', { name: 'Urlaub & Reise' }).click();
-		expect(goto).toHaveBeenCalledWith('/documents?tag=Urlaub%20%26%20Reise');
+		// Tailwind CSS isn't loaded in the vitest-browser client project, so the
+		// `z-10` that elevates the content wrapper above the stretched-link
+		// overlay anchor has no effect here — Playwright's coordinate-based
+		// click would hit the anchor instead of the tag button. Fire the click
+		// directly on the button to verify the handler logic.
+		document.querySelector<HTMLButtonElement>('button')?.click();
+		await expect
+			.poll(() => vi.mocked(goto).mock.calls[0]?.[0])
+			.toBe('/documents?tag=Urlaub%20%26%20Reise');
 	});
 
 	it('tag click does not navigate to the document detail page', async () => {