feat(annotations): dim non-active annotations when a block is focused

When activeAnnotationId is set, the active annotation stays at full
opacity with a highlight box-shadow, while all other annotations fade
to 30% opacity (300ms ease transition). When no block is focused,
all annotations show at full opacity.

Prop chain: activeAnnotationId flows from PdfViewer → AnnotationLayer.

2 new tests (RED/GREEN):
- dims non-active annotations when activeAnnotationId is set
- shows all at full opacity when no activeAnnotationId

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

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

@@ -69,6 +69,35 @@ describe('AnnotationLayer', () => {
 		expect(container.getAttribute('style')).not.toContain('cursor: crosshair');
 	});
 
+	it('dims non-active annotations when activeAnnotationId is set', async () => {
+		render(AnnotationLayer, {
+			annotations: [makeAnnotation('ann-1'), makeAnnotation('ann-2')],
+			canDraw: false,
+			color: '#00C7B1',
+			activeAnnotationId: 'ann-1',
+			onDraw: () => {}
+		});
+
+		const active = page.getByTestId('annotation-ann-1').element();
+		const dimmed = page.getByTestId('annotation-ann-2').element();
+		expect(active.style.opacity).toBe('1');
+		expect(dimmed.style.opacity).toBe('0.3');
+	});
+
+	it('shows all annotations at full opacity when no activeAnnotationId', async () => {
+		render(AnnotationLayer, {
+			annotations: [makeAnnotation('ann-1'), makeAnnotation('ann-2')],
+			canDraw: false,
+			color: '#00C7B1',
+			onDraw: () => {}
+		});
+
+		const el1 = page.getByTestId('annotation-ann-1').element();
+		const el2 = page.getByTestId('annotation-ann-2').element();
+		expect(el1.style.opacity).toBe('1');
+		expect(el2.style.opacity).toBe('1');
+	});
+
 	it('does not show delete buttons (annotations owned by blocks)', async () => {
 		render(AnnotationLayer, {
 			annotations: [makeAnnotation('ann-1')],