feat(ui): wire panelMode state with read/edit view switching

Adds TranscriptionPanelHeader and TranscriptionReadView to the
document detail page. Default mode is 'read' when blocks exist,
'edit' otherwise. Annotations dimmed in read mode.

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

frontend/src/lib/components/DocumentViewer.svelte

@@ -20,6 +20,7 @@ type Props = {
 	blockNumbers?: Record<string, number>;
 	annotationReloadKey?: number;
 	activeAnnotationId: string | null;
+	annotationsDimmed?: boolean;
 	onAnnotationClick: (id: string) => void;
 	onTranscriptionDraw?: (rect: DrawRect) => void;
 };
@@ -33,6 +34,7 @@ let {
 	blockNumbers = {},
 	annotationReloadKey = 0,
 	activeAnnotationId = $bindable(),
+	annotationsDimmed = false,
 	onAnnotationClick,
 	onTranscriptionDraw
 }: Props = $props();
@@ -90,6 +92,7 @@ let {
 			blockNumbers={blockNumbers}
 			annotationReloadKey={annotationReloadKey}
 			bind:activeAnnotationId={activeAnnotationId}
+			annotationsDimmed={annotationsDimmed}
 			onAnnotationClick={onAnnotationClick}
 			onTranscriptionDraw={onTranscriptionDraw}
 			documentFileHash={doc.fileHash ?? null}