refactor: remove legacy annotate mode — transcription replaces it

The yellow annotation+comment system is now redundant. Transcription
blocks handle the same use case (mark region → discuss) but better,
because they also produce a transcription.

Removed:
- annotateMode state and all wiring through page/topbar/viewer/pdfviewer
- Annotate/Stop annotate buttons from DocumentTopBar
- AnnotateHintStrip import and rendering
- AnnotationSidePanel from document detail page
- canAnnotate prop from DocumentTopBar
- Color picker from PdfViewer
- Comment count badges and loadCommentCounts from PdfViewer
- Delete button from AnnotationLayer (blocks own annotation lifecycle)
- dimColor prop from AnnotationLayer

Simplified:
- AnnotationLayer: only canDraw + color + onDraw + onAnnotationClick
- PdfViewer: only draws in transcribeMode with turquoise
- Clicking annotation in transcribe mode scrolls to corresponding block
- canComment derived from canWrite (no longer needs canAnnotate)

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

frontend/src/lib/components/DocumentViewer.svelte

@@ -16,10 +16,8 @@ type Props = {
 	fileUrl: string;
 	isLoading: boolean;
 	error: string;
-	annotateMode: boolean;
 	transcribeMode?: boolean;
 	activeAnnotationId: string | null;
-	activeAnnotationPage: number | null;
 	onAnnotationClick: (id: string) => void;
 	onTranscriptionDraw?: (rect: DrawRect) => void;
 };
@@ -29,10 +27,8 @@ let {
 	fileUrl,
 	isLoading,
 	error,
-	annotateMode = $bindable(),
 	transcribeMode = false,
 	activeAnnotationId = $bindable(),
-	activeAnnotationPage = $bindable(),
 	onAnnotationClick,
 	onTranscriptionDraw
 }: Props = $props();
@@ -86,10 +82,8 @@ let {
 		<PdfViewer
 			url={fileUrl}
 			documentId={doc.id}
-			bind:annotateMode={annotateMode}
 			transcribeMode={transcribeMode}
 			bind:activeAnnotationId={activeAnnotationId}
-			bind:activeAnnotationPage={activeAnnotationPage}
 			onAnnotationClick={onAnnotationClick}
 			onTranscriptionDraw={onTranscriptionDraw}
 			documentFileHash={doc.fileHash ?? null}