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/routes/documents/[id]/+page.svelte

@@ -55,6 +55,14 @@ let activeAnnotationId = $state<string | null>(null);
 
 let transcriptionBlocks = $state<TranscriptionBlockData[]>([]);
 
+const blockNumbers = $derived(
+	Object.fromEntries(
+		[...transcriptionBlocks]
+			.sort((a, b) => a.sortOrder - b.sortOrder)
+			.map((b, i) => [b.annotationId, i + 1])
+	)
+);
+
 async function loadTranscriptionBlocks() {
 	if (!doc?.id) return;
 	try {
@@ -197,6 +205,7 @@ onMount(() => {
 				isLoading={isLoading}
 				error={fileError}
 				transcribeMode={transcribeMode}
+				blockNumbers={blockNumbers}
 				bind:activeAnnotationId={activeAnnotationId}
 				onAnnotationClick={handleAnnotationClick}
 				onTranscriptionDraw={createBlockFromDraw}