feat(transcription): show block numbers on PDF annotation overlays

Add blockNumbers prop through AnnotationLayer → PdfViewer → DocumentViewer.
Each turquoise annotation rectangle now shows a numbered badge (top-left,
matching the block card number in the right panel).

Block numbers are derived from sorted transcriptionBlocks, mapped by
annotationId, creating a visual link between PDF regions and block cards.

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

frontend/src/lib/components/PdfViewer.svelte

@@ -11,6 +11,7 @@ let {
 	url,
 	documentId = '',
 	transcribeMode = false,
+	blockNumbers = {},
 	activeAnnotationId = $bindable<string | null>(null),
 	onAnnotationClick,
 	onTranscriptionDraw,
@@ -19,6 +20,7 @@ let {
 	url: string;
 	documentId?: string;
 	transcribeMode?: boolean;
+	blockNumbers?: Record<string, number>;
 	activeAnnotationId?: string | null;
 	onAnnotationClick?: (id: string) => void;
 	onTranscriptionDraw?: (rect: DrawRect) => void;
@@ -424,6 +426,7 @@ function zoomOut() {
 								annotations={visibleAnnotations.filter((a) => a.pageNumber === currentPage)}
 								canDraw={transcribeMode}
 								color={TRANSCRIPTION_COLOR}
+								blockNumbers={blockNumbers}
 								onDraw={handleDraw}
 								onAnnotationClick={handleAnnotationClick}
 							/>