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/AnnotationLayer.svelte

@@ -12,12 +12,14 @@ let {
 	annotations = [],
 	canDraw,
 	color,
+	blockNumbers = {},
 	onDraw,
 	onAnnotationClick
 }: {
 	annotations: Annotation[];
 	canDraw: boolean;
 	color: string;
+	blockNumbers?: Record<string, number>;
 	onDraw: (rect: DrawRect) => void;
 	onAnnotationClick?: (id: string) => void;
 } = $props();
@@ -125,7 +127,32 @@ const containerStyle = $derived(
 				cursor: pointer;
 				transition: background-color 0.15s ease, box-shadow 0.15s ease;
 			"
-		></div>
+		>
+			{#if blockNumbers[annotation.id]}
+				<div
+					style="
+						position: absolute;
+						top: -8px;
+						left: -8px;
+						width: 20px;
+						height: 20px;
+						border-radius: 50%;
+						background-color: {annotation.color};
+						color: white;
+						font-size: 11px;
+						font-family: sans-serif;
+						font-weight: 700;
+						display: flex;
+						align-items: center;
+						justify-content: center;
+						pointer-events: none;
+						box-shadow: 0 1px 3px rgba(0,0,0,0.3);
+					"
+				>
+					{blockNumbers[annotation.id]}
+				</div>
+			{/if}
+		</div>
 	{/each}
 
 	{#if drawRect && drawRect.width > 0}