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/PdfViewer.svelte

@@ -447,6 +447,7 @@ function zoomOut() {
 								canDraw={transcribeMode}
 								color={TRANSCRIPTION_COLOR}
 								blockNumbers={blockNumbers}
+								activeAnnotationId={activeAnnotationId}
 								onDraw={handleDraw}
 								onAnnotationClick={handleAnnotationClick}
 							/>