feat(ui): confine read-only users to the transcription read view (#697)

On the document detail page, pass canEdit={canWrite} to the panel header,
guard onModeChange so a reader can never flip to edit, and default panelMode
to 'read' for readers. Thread canAnnotate={canWrite} through DocumentViewer
to PdfViewer so the annotation layer's canDraw (which also gates delete and
resize) is off for readers — they can open and read, but not draw, edit, or
delete. The writer-only OCR status check is also skipped for readers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/document/viewer/PdfViewer.svelte

@@ -14,6 +14,7 @@ let {
 	url,
 	documentId = '',
 	transcribeMode = false,
+	canAnnotate = false,
 	blockNumbers = {},
 	annotationReloadKey = 0,
 	activeAnnotationId = $bindable<string | null>(null),
@@ -28,6 +29,7 @@ let {
 	url: string;
 	documentId?: string;
 	transcribeMode?: boolean;
+	canAnnotate?: boolean;
 	blockNumbers?: Record<string, number>;
 	annotationReloadKey?: number;
 	activeAnnotationId?: string | null;
@@ -262,7 +264,7 @@ function handleAnnotationClick(id: string) {
 								annotations={visibleAnnotations.filter(
 									(a) => a.pageNumber === renderer.currentPage
 								)}
-								canDraw={transcribeMode}
+								canDraw={transcribeMode && canAnnotate}
 								color={TRANSCRIPTION_COLOR}
 								blockNumbers={blockNumbers}
 								activeAnnotationId={activeAnnotationId}