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/TranscriptionBlock.svelte

@@ -1,15 +1,19 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
+import CommentThread from './CommentThread.svelte';
 
 type SaveState = 'idle' | 'saving' | 'saved' | 'fading' | 'error';
 
 type Props = {
 	blockId: string;
+	documentId: string;
 	blockNumber: number;
 	text: string;
 	label: string | null;
 	active: boolean;
 	saveState: SaveState;
+	canComment: boolean;
+	currentUserId: string | null;
 	onTextChange: (text: string) => void;
 	onFocus: () => void;
 	onDeleteClick: () => void;
@@ -18,22 +22,30 @@ type Props = {
 
 let {
 	blockId,
+	documentId,
 	blockNumber,
 	text,
 	label = null,
 	active,
 	saveState,
+	canComment,
+	currentUserId,
 	onTextChange,
 	onFocus,
 	onDeleteClick,
 	onRetry
 }: Props = $props();
 
+let commentOpen = $state(false);
+let selectedQuote = $state<string | null>(null);
+let textareaEl = $state<HTMLTextAreaElement | null>(null);
+
 let leftBorderClass = $derived(
 	saveState === 'error' ? 'border-l-2 border-error' : active ? 'border-l-2 border-turquoise' : ''
 );
 
 function autoresize(node: HTMLTextAreaElement) {
+	textareaEl = node;
 	function resize() {
 		node.style.height = 'auto';
 		node.style.height = `${node.scrollHeight}px`;
@@ -45,7 +57,9 @@ function autoresize(node: HTMLTextAreaElement) {
 		update() {
 			resize();
 		},
-		destroy() {}
+		destroy() {
+			textareaEl = null;
+		}
 	};
 }
 
@@ -59,6 +73,19 @@ function handleDelete() {
 		onDeleteClick();
 	}
 }
+
+function captureSelectionAndOpenComments() {
+	if (textareaEl) {
+		const start = textareaEl.selectionStart;
+		const end = textareaEl.selectionEnd;
+		if (start !== end) {
+			selectedQuote = textareaEl.value.substring(start, end);
+		} else {
+			selectedQuote = null;
+		}
+	}
+	commentOpen = true;
+}
 </script>
 
 <div
@@ -93,7 +120,22 @@ function handleDelete() {
 		></textarea>
 
 		<!-- Footer -->
-		<div class="flex items-center justify-end border-t border-line pt-2">
+		<div class="flex items-center justify-between border-t border-line pt-2">
+			<div class="flex flex-col gap-1">
+				<button
+					type="button"
+					class="text-xs font-medium text-ink-2 transition-colors hover:text-ink"
+					onclick={captureSelectionAndOpenComments}
+				>
+					{m.transcription_block_comment_btn()}
+				</button>
+				{#if active}
+					<span class="text-xs text-ink-3">
+						{m.transcription_block_quote_hint()}
+					</span>
+				{/if}
+			</div>
+
 			<div class="flex items-center gap-2">
 				<!-- Save state indicator -->
 				{#if saveState === 'saving'}
@@ -143,5 +185,35 @@ function handleDelete() {
 				</button>
 			</div>
 		</div>
+
+		<!-- Comment thread (expandable) -->
+		{#if commentOpen}
+			<div class="mt-3 border-t border-line pt-3">
+				<div class="mb-2 flex items-center justify-between">
+					<span class="font-sans text-xs font-semibold text-ink-2">
+						{m.comment_section_title()}
+					</span>
+					<button
+						type="button"
+						class="font-sans text-xs text-ink-3 transition-colors hover:text-ink"
+						onclick={() => {
+							commentOpen = false;
+							selectedQuote = null;
+						}}
+					>
+						{m.comment_panel_close()}
+					</button>
+				</div>
+				<CommentThread
+					documentId={documentId}
+					blockId={blockId}
+					loadOnMount={true}
+					canComment={canComment}
+					currentUserId={currentUserId}
+					canAdmin={false}
+					quotedText={selectedQuote}
+				/>
+			</div>
+		{/if}
 	</div>
 </div>