fix(tests): fix 27 failing frontend unit tests

Six categories of breakage:

1. date.ts — add formatGermanDateInput(raw: string): string as a pure
   function covering both digit-stream auto-dot and manual-dot-with-padding
   modes. Refactor handleGermanDateInput to delegate to it. Fixes 16 failures
   in date.spec.ts where the function was imported but didn't exist.

2. Admin layout specs (groups/tags/users) — $effect fires on initial mount
   with manualCollapse=false, so the spy captured 'false' before the click's
   effect ran. Fix: move spy setup after render(), add await setTimeout(0) to
   flush Svelte effects before asserting.

3. DashboardMentions — component now renders a persistent
   "Benachrichtigungsverlauf ansehen" link, making getByRole('link') strict-
   mode violations. Fix: scope link queries to the actor name, and check
   absence of the actor link (not all links) in the no-documentId test.

4. Conversations page — empty-state copy changed from "Wählen Sie zwei
   Personen aus" to "Korrespondenz durchsuchen". Update the test.

5. Login page — AuthHeader adds a second aria-label="Familienarchiv" link.
   Use .first() to avoid strict-mode violation.

6. Persons page — alias is rendered with German quotation marks „…" not
   straight quotes "…". Update the test string.

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

frontend/src/routes/login/page.svelte.spec.ts

@@ -10,7 +10,9 @@ afterEach(cleanup);
 describe('Login page – rendering', () => {
 	it('renders the page title', async () => {
 		render(LoginPage, {});
-		await expect.element(page.getByRole('link', { name: 'Familienarchiv' })).toBeInTheDocument();
+		await expect
+			.element(page.getByRole('link', { name: 'Familienarchiv' }).first())
+			.toBeInTheDocument();
 		await page.screenshot({ path: 'test-results/screenshots/login-default.png' });
 	});