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

@@ -71,7 +71,7 @@ const ocrJob = createOcrJob({
 	onJobFinished: async () => {
 		await transcription.load();
 		transcription.bumpAnnotationReloadKey();
-		panelMode = transcription.hasBlocks ? 'read' : 'edit';
+		panelMode = canWrite && !transcription.hasBlocks ? 'edit' : 'read';
 	}
 });
 
@@ -148,10 +148,10 @@ $effect(() => {
 			if (skipInitialPanelMode) {
 				skipInitialPanelMode = false;
 			} else {
-				panelMode = transcription.hasBlocks ? 'read' : 'edit';
+				panelMode = canWrite && !transcription.hasBlocks ? 'edit' : 'read';
 			}
 		});
-		ocrJob.checkStatus();
+		if (canWrite) ocrJob.checkStatus();
 	}
 });
 
@@ -252,6 +252,7 @@ onMount(() => {
 				isLoading={fileLoader.isLoading}
 				error={fileLoader.fileError}
 				transcribeMode={transcribeMode && !ocrJob.running}
+				canAnnotate={canWrite}
 				blockNumbers={transcription.blockNumbers}
 				annotationReloadKey={transcription.annotationReloadKey}
 				annotationsDimmed={transcribeMode && panelMode === 'read'}
@@ -293,7 +294,10 @@ onMount(() => {
 					hasBlocks={transcription.hasBlocks}
 					blockCount={transcription.blocks.length}
 					lastEditedAt={transcription.lastEditedAt}
-					onModeChange={(newMode) => (panelMode = newMode)}
+					canEdit={canWrite}
+					onModeChange={(newMode) => {
+						if (canWrite) panelMode = newMode;
+					}}
 					onClose={() => (transcribeMode = false)}
 				/>
 				<div class="flex-1 overflow-y-auto">