feat(transcription): swap plain textarea for PersonMentionEditor and thread mentionedPersons through autosave

- TranscriptionBlockData now carries mentionedPersons (matches backend
  schema added in PR-A).
- useBlockAutoSave.saveFn signature widens to (blockId, text, mentions);
  pendingMentions is tracked alongside pendingTexts and is preserved on
  failure so a retry resends the in-flight payload (B12).
- TranscriptionBlock.svelte renders <PersonMentionEditor>, exposing the
  textarea node back through a captureTextarea callback so the existing
  quote-selection feature still works.
- saveBlock in routes/documents/[id]/+page.svelte forwards mentions on
  PUT.
- flushOnUnload sends mentions in the keepalive payload too.

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

frontend/src/routes/documents/[id]/+page.svelte

@@ -88,11 +88,15 @@ async function loadTranscriptionBlocks() {
 	}
 }
 
-async function saveBlock(blockId: string, text: string) {
+async function saveBlock(
+	blockId: string,
+	text: string,
+	mentionedPersons: import('$lib/types').PersonMention[]
+) {
 	const res = await fetch(`/api/documents/${doc.id}/transcription-blocks/${blockId}`, {
 		method: 'PUT',
 		headers: { 'Content-Type': 'application/json' },
-		body: JSON.stringify({ text })
+		body: JSON.stringify({ text, mentionedPersons })
 	});
 	if (!res.ok) throw new Error('Save failed');
 	const updated = await res.json();