test(ui): add flash-highlight class tests for TranscriptionReadView

Verifies highlightBlockId applies and removes the flash-highlight
CSS class correctly.

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

frontend/src/lib/components/TranscriptionReadView.svelte.test.ts

@@ -108,6 +108,28 @@ describe('TranscriptionReadView', () => {
 		expect(paragraphs[1].getAttribute('data-block-id')).toBe('b1');
 	});
 
+	it('should apply flash-highlight class when highlightBlockId matches', async () => {
+		render(TranscriptionReadView, {
+			blocks: [blocks[0]],
+			onParagraphClick: () => {},
+			highlightBlockId: 'b1'
+		});
+
+		const el = document.querySelector('[data-block-id="b1"]')!;
+		expect(el.classList.contains('flash-highlight')).toBe(true);
+	});
+
+	it('should not apply flash-highlight class when highlightBlockId does not match', async () => {
+		render(TranscriptionReadView, {
+			blocks: [blocks[0]],
+			onParagraphClick: () => {},
+			highlightBlockId: 'other-id'
+		});
+
+		const el = document.querySelector('[data-block-id="b1"]')!;
+		expect(el.classList.contains('flash-highlight')).toBe(false);
+	});
+
 	it('should render empty state when no blocks', async () => {
 		render(TranscriptionReadView, {
 			blocks: [],