test(person-mention): move i18n test to its own describe block

Move `transcription_block_placeholder contains @ mention trigger` out of
`describe('PersonMentionEditor — placeholder behavior')` into a new
`describe('PersonMentionEditor — i18n message content')` block so each
describe group has a single, clear responsibility.

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

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

@@ -394,7 +394,11 @@ describe('PersonMentionEditor — placeholder behavior', () => {
 			expect(inner!.hasAttribute('data-placeholder')).toBe(false);
 		});
 	});
+});
 
+// ─── i18n message content ─────────────────────────────────────────────────────
+
+describe('PersonMentionEditor — i18n message content', () => {
 	it('transcription_block_placeholder contains @ mention trigger for discoverability', () => {
 		expect(m.transcription_block_placeholder()).toContain('@');
 	});