feat(transcription): add block-level comment threads with quote support

TranscriptionBlock.svelte:
- "Kommentieren" button opens expandable comment thread per block
- Text selection in textarea captured as quoted text (> "...") prefix
- Quote hint "Text markieren für Zitat" shown when block is active/focused
- Comment thread uses existing CommentThread with blockId prop

CommentThread.svelte:
- Add blockId prop for block-level comments URL routing
- Add quotedText prop — pre-fills comment input with markdown blockquote
- commentsBase now supports 3 URL patterns: document, annotation, block

TranscriptionEditView.svelte:
- Pass canComment + currentUserId through to block components

3 new frontend tests:
- Kommentieren button present
- Quote hint shown when active
- Quote hint hidden when inactive

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/TranscriptionBlock.svelte.spec.ts

@@ -8,11 +8,14 @@ afterEach(cleanup);
 function renderBlock(overrides: Record<string, unknown> = {}) {
 	return render(TranscriptionBlock, {
 		blockId: 'block-1',
+		documentId: 'doc-1',
 		blockNumber: 3,
 		text: 'Liebe Mutter,',
 		label: null,
 		active: false,
 		saveState: 'idle' as const,
+		canComment: true,
+		currentUserId: 'user-1',
 		onTextChange: vi.fn(),
 		onFocus: vi.fn(),
 		onDeleteClick: vi.fn(),
@@ -111,4 +114,21 @@ describe('TranscriptionBlock — interactions', () => {
 		await textarea.click();
 		expect(onFocus).toHaveBeenCalled();
 	});
+
+	it('shows Kommentieren button that opens comment thread', async () => {
+		renderBlock();
+		const btn = page.getByText('Kommentieren');
+		await expect.element(btn).toBeInTheDocument();
+	});
+
+	it('shows quote hint when block is active', async () => {
+		renderBlock({ active: true });
+		await expect.element(page.getByText('Text markieren für Zitat')).toBeInTheDocument();
+	});
+
+	it('hides quote hint when block is not active', async () => {
+		renderBlock({ active: false });
+		const hint = page.getByText('Text markieren für Zitat');
+		await expect.element(hint).not.toBeInTheDocument();
+	});
 });