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.spec.ts

@@ -15,7 +15,8 @@ const block1 = {
 	sortOrder: 0,
 	version: 0,
 	source: 'MANUAL' as const,
-	reviewed: false
+	reviewed: false,
+	mentionedPersons: []
 };
 const block2 = {
 	id: 'b2',
@@ -26,7 +27,8 @@ const block2 = {
 	sortOrder: 1,
 	version: 0,
 	source: 'OCR' as const,
-	reviewed: true
+	reviewed: true,
+	mentionedPersons: []
 };
 
 function renderView(overrides: Record<string, unknown> = {}, service = createConfirmService()) {
@@ -141,7 +143,28 @@ describe('TranscriptionEditView — auto-save debounce', () => {
 		vi.advanceTimersByTime(1500);
 		await vi.runAllTimersAsync();
 
-		expect(onSaveBlock).toHaveBeenCalledWith('b1', 'Neue Zeile');
+		expect(onSaveBlock).toHaveBeenCalledWith('b1', 'Neue Zeile', []);
+		vi.useRealTimers();
+	});
+
+	it('passes the block mentionedPersons array as the 3rd save argument', async () => {
+		vi.useFakeTimers();
+		const onSaveBlock = vi.fn().mockResolvedValue(undefined);
+		const blockWithMention = {
+			...block1,
+			mentionedPersons: [{ personId: 'p-aug', displayName: 'Auguste Raddatz' }]
+		};
+		renderView({ blocks: [blockWithMention], onSaveBlock });
+
+		const textarea = page.getByRole('textbox').first();
+		await textarea.fill('Hallo @Auguste Raddatz');
+
+		vi.advanceTimersByTime(1500);
+		await vi.runAllTimersAsync();
+
+		expect(onSaveBlock).toHaveBeenCalledWith('b1', 'Hallo @Auguste Raddatz', [
+			{ personId: 'p-aug', displayName: 'Auguste Raddatz' }
+		]);
 		vi.useRealTimers();
 	});
 
@@ -165,7 +188,7 @@ describe('TranscriptionEditView — auto-save debounce', () => {
 
 		// Only one save with the final value
 		expect(onSaveBlock).toHaveBeenCalledTimes(1);
-		expect(onSaveBlock).toHaveBeenCalledWith('b1', 'Second');
+		expect(onSaveBlock).toHaveBeenCalledWith('b1', 'Second', []);
 		vi.useRealTimers();
 	});
 });
@@ -220,7 +243,7 @@ describe('TranscriptionEditView — flush on blur', () => {
 		el.dispatchEvent(new FocusEvent('blur', { bubbles: true }));
 
 		await vi.runAllTimersAsync();
-		expect(onSaveBlock).toHaveBeenCalledWith('b1', 'Blur text');
+		expect(onSaveBlock).toHaveBeenCalledWith('b1', 'Blur text', []);
 		vi.useRealTimers();
 	});
 });