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.test.ts

@@ -86,6 +86,38 @@ describe('PdfViewer — loaded state', () => {
 		}
 	});
 
+	it('makes the annotation surface drawable (crosshair) when transcribeMode and canAnnotate', async () => {
+		render(PdfViewer, {
+			url: '/api/documents/test/file',
+			documentId: 'test',
+			transcribeMode: true,
+			canAnnotate: true,
+			libLoader: makeFakeLibLoader()
+		});
+
+		await vi.waitFor(() => {
+			const surface = document.querySelector('[role="presentation"]');
+			expect(surface).not.toBeNull();
+			expect(surface?.getAttribute('style') ?? '').toContain('crosshair');
+		});
+	});
+
+	it('does not make the annotation surface drawable when canAnnotate is false (read-only user)', async () => {
+		render(PdfViewer, {
+			url: '/api/documents/test/file',
+			documentId: 'test',
+			transcribeMode: true,
+			canAnnotate: false,
+			libLoader: makeFakeLibLoader()
+		});
+
+		await vi.waitFor(() => {
+			expect(document.querySelector('.bg-pdf-bg')).not.toBeNull();
+		});
+		const surface = document.querySelector('[role="presentation"]');
+		expect(surface?.getAttribute('style') ?? '').not.toContain('crosshair');
+	});
+
 	it('renders the canvas region when documentFileHash is provided', async () => {
 		render(PdfViewer, {
 			url: '/api/documents/test/file',