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/TranscriptionBlock.test-host.svelte

@@ -1,21 +1,24 @@
 <script lang="ts">
 import { provideConfirmService, type ConfirmService } from '$lib/services/confirm.svelte.js';
 import TranscriptionBlock from './TranscriptionBlock.svelte';
+import type { PersonMention } from '$lib/types';
 
 type BlockProps = {
 	blockId: string;
 	documentId: string;
 	blockNumber: number;
 	text: string;
+	mentionedPersons?: PersonMention[];
 	label: string | null;
 	active: boolean;
 	saveState: 'idle' | 'saving' | 'saved' | 'fading' | 'error';
 	canComment: boolean;
 	currentUserId: string | null;
-	onTextChange: (text: string) => void;
+	onTextChange: (text: string, mentionedPersons: PersonMention[]) => void;
 	onFocus: () => void;
 	onDeleteClick: () => void;
 	onRetry: () => void;
+	onReviewToggle?: () => void;
 	onMoveUp?: () => void;
 	onMoveDown?: () => void;
 	isFirst?: boolean;
@@ -24,13 +27,22 @@ type BlockProps = {
 
 let {
 	onServiceReady,
+	mentionedPersons = [],
+	reviewed = false,
+	onReviewToggle = () => {},
 	...blockProps
 }: BlockProps & {
 	onServiceReady: (s: ConfirmService) => void;
+	reviewed?: boolean;
 } = $props();
 
 const service = provideConfirmService();
 onServiceReady(service);
 </script>
 
-<TranscriptionBlock {...blockProps} />
+<TranscriptionBlock
+	{...blockProps}
+	mentionedPersons={mentionedPersons}
+	reviewed={reviewed}
+	onReviewToggle={onReviewToggle}
+/>