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.spec.ts

@@ -16,6 +16,7 @@ function renderBlock(overrides: Record<string, unknown> = {}) {
 		saveState: 'idle' as const,
 		canComment: true,
 		currentUserId: 'user-1',
+		commentCount: 0,
 		onTextChange: vi.fn(),
 		onFocus: vi.fn(),
 		onDeleteClick: vi.fn(),