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/components/TranscriptionEditView.svelte

@@ -3,7 +3,7 @@ import { m } from '$lib/paraglide/messages.js';
 import TranscriptionBlock from './TranscriptionBlock.svelte';
 import OcrTrigger from './OcrTrigger.svelte';
 import TranscribeCoachEmptyState from './TranscribeCoachEmptyState.svelte';
-import type { TranscriptionBlockData } from '$lib/types';
+import type { PersonMention, TranscriptionBlockData } from '$lib/types';
 import { createBlockAutoSave } from '$lib/hooks/useBlockAutoSave.svelte';
 import { createBlockDragDrop } from '$lib/hooks/useBlockDragDrop.svelte';
 
@@ -16,7 +16,7 @@ type Props = {
 	storedScriptType?: string;
 	canRunOcr?: boolean;
 	onBlockFocus: (blockId: string) => void;
-	onSaveBlock: (blockId: string, text: string) => Promise<void>;
+	onSaveBlock: (blockId: string, text: string, mentionedPersons: PersonMention[]) => Promise<void>;
 	onDeleteBlock: (blockId: string) => Promise<void>;
 	onReviewToggle: (blockId: string) => Promise<void>;
 	onMarkAllReviewed?: () => Promise<void>;
@@ -245,16 +245,19 @@ async function handleLabelToggle(label: string) {
 							documentId={documentId}
 							blockNumber={i + 1}
 							text={block.text}
+							mentionedPersons={block.mentionedPersons ?? []}
 							label={block.label}
 							active={activeBlockId === block.id}
 							reviewed={block.reviewed ?? false}
 							saveState={autoSave.getSaveState(block.id)}
 							canComment={canComment}
 							currentUserId={currentUserId}
-							onTextChange={(text) => autoSave.handleTextChange(block.id, text)}
+							onTextChange={(text, mentions) =>
+								autoSave.handleTextChange(block.id, text, mentions)}
 							onFocus={() => handleFocus(block.id)}
 							onDeleteClick={() => handleDelete(block.id)}
-							onRetry={() => autoSave.handleRetry(block.id, block.text)}
+							onRetry={() =>
+								autoSave.handleRetry(block.id, block.text, block.mentionedPersons ?? [])}
 							onReviewToggle={() => onReviewToggle(block.id)}
 							onMoveUp={() => handleMoveUp(block.id)}
 							onMoveDown={() => handleMoveDown(block.id)}