a11y(transcription): hide visible @mention empty-state from AT and fold empty-query check

Round-4 polish from Leonie (S-2), Felix (#3), Sara (#4):
- Add aria-hidden="true" to the visible empty-state <p> so VoiceOver does
  not double-announce — the persistent sr-only live region is now the
  sole AT source of truth (NVDA already de-duped, VoiceOver did not).
- Extract `searchQuery.trim() === ''` into an `isQueryEmpty` $derived;
  both the announcer branch and the visible empty-state branch now read
  from the single intent-named alias.
- Cover the singular branch of the persistent live region (1 item ->
  "1 Person gefunden" / "1 person found" / "1 persona encontrada").
  Plural was already covered; this closes the missing-branch gap.
- Extend the existing "no aria-live on visible <p>" test to also assert
  aria-hidden="true" so a regression on the AT-source-of-truth contract
  goes red immediately.

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

frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts

@@ -198,7 +198,25 @@ describe('MentionDropdown — search input', () => {
 		expect(live!.textContent ?? '').toContain('3');
 	});
 
-	it('keeps the visible empty-state copy without its own aria-live (the persistent live region announces; Leonie #3 on PR #629)', async () => {
+	it('announces the singular form when exactly one item is present (Sara #4 on PR #629)', async () => {
+		render(MentionDropdown, {
+			props: {
+				model: baseModel({
+					items: [makePerson('p1', 'Anna Schmidt')]
+				})
+			}
+		});
+
+		const listbox = document.querySelector('[role="listbox"]');
+		expect(listbox).not.toBeNull();
+		const live = listbox!.querySelector('p[aria-live="polite"]');
+		expect(live).not.toBeNull();
+		// Singular branch — "1 Person gefunden" / "1 person found" / "1 persona encontrada"
+		// (locale-dependent; resolved via the Paraglide message helper).
+		expect(live!.textContent ?? '').toContain(m.person_mention_results_count_singular());
+	});
+
+	it('keeps the visible empty-state copy without its own aria-live and hides it from AT (Leonie #3 on PR #629 round 3; Leonie S-2 round 4)', async () => {
 		render(MentionDropdown, { props: { model: baseModel(), editorQuery: 'WdG' } });
 
 		// Visible empty-state <p> exists with the empty-result copy ...
@@ -208,6 +226,10 @@ describe('MentionDropdown — search input', () => {
 		// ... but it must NOT carry its own aria-live (the persistent sr-only
 		// region above the conditional is the announcer now).
 		expect(empty!.hasAttribute('aria-live')).toBe(false);
+		// ... and it MUST be hidden from screen readers via aria-hidden="true"
+		// so VoiceOver does not double-announce (the persistent sr-only region
+		// is the sole AT source of truth). Leonie S-2 on PR #629 round 4.
+		expect(empty!.getAttribute('aria-hidden')).toBe('true');
 	});
 
 	it('renders the magnifier icon at h-5 w-5 with text-ink-2 (Leonie BLOCKER on PR #629)', async () => {