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/TranscriptionEditView.svelte

@@ -9,12 +9,22 @@ type SaveState = 'idle' | 'saving' | 'saved' | 'fading' | 'error';
 type Props = {
 	documentId: string;
 	blocks: TranscriptionBlockData[];
+	canComment: boolean;
+	currentUserId: string | null;
 	onBlockFocus: (blockId: string) => void;
 	onSaveBlock: (blockId: string, text: string) => Promise<void>;
 	onDeleteBlock: (blockId: string) => Promise<void>;
 };
 
-let { documentId, blocks, onBlockFocus, onSaveBlock, onDeleteBlock }: Props = $props();
+let {
+	documentId,
+	blocks,
+	canComment,
+	currentUserId,
+	onBlockFocus,
+	onSaveBlock,
+	onDeleteBlock
+}: Props = $props();
 
 let activeBlockId: string | null = $state(null);
 let saveStates = new SvelteMap<string, SaveState>();
@@ -149,11 +159,14 @@ $effect(() => {
 				<div onblur={handleBlur}>
 					<TranscriptionBlock
 						blockId={block.id}
+						documentId={documentId}
 						blockNumber={i + 1}
 						text={block.text}
 						label={block.label}
 						active={activeBlockId === block.id}
 						saveState={getSaveState(block.id)}
+						canComment={canComment}
+						currentUserId={currentUserId}
 						onTextChange={(text) => handleTextChange(block.id, text)}
 						onFocus={() => handleFocus(block.id)}
 						onDeleteClick={() => handleDelete(block.id)}