fix(transcription): auto-expand comment thread when block has comments

Comments were only shown after clicking "Kommentieren". Now:
- Load comment counts per block when blocks are loaded
- Pass commentCount prop to TranscriptionBlock
- If commentCount > 0, the comment thread is expanded by default
- If commentCount is 0, thread stays collapsed behind the button

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

frontend/src/lib/components/TranscriptionBlock.svelte

@@ -14,6 +14,7 @@ type Props = {
 	saveState: SaveState;
 	canComment: boolean;
 	currentUserId: string | null;
+	commentCount: number;
 	onTextChange: (text: string) => void;
 	onFocus: () => void;
 	onDeleteClick: () => void;
@@ -30,6 +31,7 @@ let {
 	saveState,
 	canComment,
 	currentUserId,
+	commentCount,
 	onTextChange,
 	onFocus,
 	onDeleteClick,
@@ -37,7 +39,7 @@ let {
 }: Props = $props();
 
 let localText = $state(text);
-let commentOpen = $state(false);
+let commentOpen = $state(commentCount > 0);
 let selectedQuote = $state<string | null>(null);
 let textareaEl = $state<HTMLTextAreaElement | null>(null);