feat(transcription): add block-level comment threads with quote support

TranscriptionBlock.svelte:
- "Kommentieren" button opens expandable comment thread per block
- Text selection in textarea captured as quoted text (> "...") prefix
- Quote hint "Text markieren für Zitat" shown when block is active/focused
- Comment thread uses existing CommentThread with blockId prop

CommentThread.svelte:
- Add blockId prop for block-level comments URL routing
- Add quotedText prop — pre-fills comment input with markdown blockquote
- commentsBase now supports 3 URL patterns: document, annotation, block

TranscriptionEditView.svelte:
- Pass canComment + currentUserId through to block components

3 new frontend tests:
- Kommentieren button present
- Quote hint shown when active
- Quote hint hidden when inactive

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

frontend/src/lib/components/CommentThread.svelte

@@ -9,24 +9,28 @@ import type { MentionDTO } from '$lib/types';
 type Props = {
 	documentId: string;
 	annotationId?: string | null;
+	blockId?: string | null;
 	initialComments?: Comment[];
 	loadOnMount?: boolean;
 	canComment: boolean;
 	currentUserId: string | null;
 	canAdmin: boolean;
 	targetCommentId?: string | null;
+	quotedText?: string | null;
 	onCountChange?: (count: number) => void;
 };
 
 let {
 	documentId,
 	annotationId = null,
+	blockId = null,
 	initialComments = [],
 	loadOnMount = false,
 	canComment,
 	currentUserId,
 	canAdmin,
 	targetCommentId = null,
+	quotedText = null,
 	onCountChange
 }: Props = $props();
 
@@ -43,11 +47,20 @@ let replyMentionCandidates: MentionDTO[] = $state([]);
 let editMentionCandidates: MentionDTO[] = $state([]);
 
 const commentsBase = $derived(
-	annotationId
-		? `/api/documents/${documentId}/annotations/${annotationId}/comments`
-		: `/api/documents/${documentId}/comments`
+	blockId
+		? `/api/documents/${documentId}/transcription-blocks/${blockId}/comments`
+		: annotationId
+			? `/api/documents/${documentId}/annotations/${annotationId}/comments`
+			: `/api/documents/${documentId}/comments`
 );
 
+// Pre-fill comment box with quoted text when selection changes
+$effect(() => {
+	if (quotedText && quotedText.trim()) {
+		newText = `> "${quotedText}"\n\n`;
+	}
+});
+
 function timeAgo(iso: string): string {
 	const diff = Date.now() - new Date(iso).getTime();
 	const minutes = Math.floor(diff / 60000);