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/admin/users/layout.svelte.spec.ts

@@ -131,9 +131,10 @@ describe('UsersListPanel — collapse toggle', () => {
 	});
 
 	it('persists collapse state using the users-specific localStorage key', async () => {
-		const setSpy = vi.spyOn(Storage.prototype, 'setItem');
 		render(UsersListPanel, { users });
+		const setSpy = vi.spyOn(Storage.prototype, 'setItem');
 		document.querySelector<HTMLButtonElement>('[aria-label="Liste einklappen"]')!.click();
+		await new Promise((r) => setTimeout(r, 0));
 		expect(setSpy).toHaveBeenCalledWith('admin_users_list_collapsed', 'true');
 		setSpy.mockRestore();
 	});