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/lib/hooks/__tests__/useBlockDragDrop.svelte.test.ts

@@ -12,7 +12,8 @@ function makeBlock(id: string, sortOrder: number): TranscriptionBlockData {
 		sortOrder,
 		version: 1,
 		source: 'MANUAL',
-		reviewed: false
+		reviewed: false,
+		mentionedPersons: []
 	};
 }