refactor(types): extract TranscriptionBlockData to shared types

Move duplicated type definition from TranscriptionEditView.svelte
and +page.svelte into $lib/types.ts for single source of truth.

Fixes @Felix: "Consider extracting the TranscriptionBlockData type"

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

frontend/src/routes/documents/[id]/+page.svelte

@@ -5,6 +5,7 @@ import DocumentTopBar from '$lib/components/DocumentTopBar.svelte';
 import DocumentViewer from '$lib/components/DocumentViewer.svelte';
 import AnnotationSidePanel from '$lib/components/AnnotationSidePanel.svelte';
 import TranscriptionEditView from '$lib/components/TranscriptionEditView.svelte';
+import type { TranscriptionBlockData } from '$lib/types';
 
 let { data } = $props();
 
@@ -65,16 +66,6 @@ let activeAnnotationPage = $state<number | null>(null);
 
 // ── Transcription blocks ─────────────────────────────────────────────────────
 
-type TranscriptionBlockData = {
-	id: string;
-	annotationId: string;
-	documentId: string;
-	text: string;
-	label: string | null;
-	sortOrder: number;
-	version: number;
-};
-
 let transcriptionBlocks = $state<TranscriptionBlockData[]>([]);
 
 async function loadTranscriptionBlocks() {