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/TranscriptionReadView.svelte.test.ts

@@ -12,7 +12,10 @@ const blocks: TranscriptionBlockData[] = [
 		text: 'First paragraph text.',
 		label: null,
 		sortOrder: 1,
-		version: 1
+		version: 1,
+		source: 'MANUAL',
+		reviewed: false,
+		mentionedPersons: []
 	},
 	{
 		id: 'b2',
@@ -49,7 +52,10 @@ describe('TranscriptionReadView', () => {
 					text: 'Text before [unleserlich] text after',
 					label: null,
 					sortOrder: 1,
-					version: 1
+					version: 1,
+					source: 'MANUAL',
+					reviewed: false,
+					mentionedPersons: []
 				}
 			],
 			onParagraphClick: () => {}
@@ -71,7 +77,10 @@ describe('TranscriptionReadView', () => {
 					text: 'Some [...] text',
 					label: null,
 					sortOrder: 1,
-					version: 1
+					version: 1,
+					source: 'MANUAL',
+					reviewed: false,
+					mentionedPersons: []
 				}
 			],
 			onParagraphClick: () => {}