feat(person-mention): update transcription placeholder with @mention discoverability hint

Replaces the generic "Type text here..." placeholder in TranscriptionBlock
with copy that teaches the @Name trigger inline (Leonie Voss design review,
issue #370). No new DOM, no new i18n keys — just the three existing
`transcription_block_placeholder` strings.

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

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

@@ -10,6 +10,7 @@ import { cleanup, render } from 'vitest-browser-svelte';
 import { page, userEvent } from 'vitest/browser';
 import PersonMentionEditorHost from './PersonMentionEditor.test-host.svelte';
 import type { components } from '$lib/generated/api';
+import { m } from '$lib/paraglide/messages.js';
 
 type Person = components['schemas']['Person'];
 type PersonMention = components['schemas']['PersonMention'];
@@ -393,6 +394,10 @@ describe('PersonMentionEditor — placeholder behavior', () => {
 			expect(inner!.hasAttribute('data-placeholder')).toBe(false);
 		});
 	});
+
+	it('transcription_block_placeholder contains @ mention trigger for discoverability', () => {
+		expect(m.transcription_block_placeholder()).toContain('@');
+	});
 });
 
 // ─── Touch target (WCAG 2.2 AA) ──────────────────────────────────────────────