feat(transcription): enable drawing turquoise rectangles on PDF to create blocks

- AnnotationLayer: add dimColor prop — annotations matching dim color
  render at 30% opacity with pointer-events disabled (300ms transition)
- PdfViewer: add transcribeMode prop, derived drawingEnabled/drawColor;
  in transcribe mode draws with turquoise (#00C7B1), routes draw events
  to onTranscriptionDraw callback instead of annotation endpoint
- DocumentViewer: pass through transcribeMode + onTranscriptionDraw
- Document detail page: createBlockFromDraw() POSTs to transcription
  blocks API on draw completion, adds created block to list
- Mode-based dimming: yellow annotations dim in transcribe mode,
  turquoise annotations dim in annotate mode

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

frontend/src/lib/components/AnnotationLayer.svelte

@@ -12,6 +12,7 @@ let {
 	annotations = [],
 	canAnnotate,
 	color,
+	dimColor,
 	onDraw,
 	onDelete,
 	commentCounts,
@@ -20,12 +21,18 @@ let {
 	annotations: Annotation[];
 	canAnnotate: boolean;
 	color: string;
+	dimColor?: string;
 	onDraw: (rect: { x: number; y: number; width: number; height: number }) => void;
 	onDelete: (id: string) => void;
 	commentCounts?: Record<string, number>;
 	onAnnotationClick?: (id: string) => void;
 } = $props();
 
+function isDimmed(annotation: Annotation): boolean {
+	if (!dimColor) return false;
+	return annotation.color.toLowerCase() === dimColor.toLowerCase();
+}
+
 let drawStart = $state<{ x: number; y: number } | null>(null);
 let drawRect = $state<DrawRect | null>(null);
 
@@ -123,8 +130,9 @@ const containerStyle = $derived(
 				height: {annotation.height * 100}%;
 				background-color: {hexToRgba(annotation.color, hoveredId === annotation.id ? 0.5 : 0.3)};
 				box-shadow: {hoveredId === annotation.id ? `inset 0 0 0 2px ${hexToRgba(annotation.color, 0.8)}` : 'none'};
-				pointer-events: auto;
-				transition: background-color 0.15s ease, box-shadow 0.15s ease;
+				opacity: {isDimmed(annotation) ? 0.3 : 1};
+				pointer-events: {isDimmed(annotation) ? 'none' : 'auto'};
+				transition: background-color 0.15s ease, box-shadow 0.15s ease, opacity 0.3s ease;
 				{onAnnotationClick && !canAnnotate ? 'cursor: pointer;' : ''}
 			"
 		>