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/DocumentViewer.svelte

@@ -17,6 +17,7 @@ type Props = {
 	isLoading: boolean;
 	error: string;
 	transcribeMode?: boolean;
+	blockNumbers?: Record<string, number>;
 	activeAnnotationId: string | null;
 	onAnnotationClick: (id: string) => void;
 	onTranscriptionDraw?: (rect: DrawRect) => void;
@@ -28,6 +29,7 @@ let {
 	isLoading,
 	error,
 	transcribeMode = false,
+	blockNumbers = {},
 	activeAnnotationId = $bindable(),
 	onAnnotationClick,
 	onTranscriptionDraw
@@ -83,6 +85,7 @@ let {
 			url={fileUrl}
 			documentId={doc.id}
 			transcribeMode={transcribeMode}
+			blockNumbers={blockNumbers}
 			bind:activeAnnotationId={activeAnnotationId}
 			onAnnotationClick={onAnnotationClick}
 			onTranscriptionDraw={onTranscriptionDraw}