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/lib/components/TranscriptionEditView.svelte

@@ -2,16 +2,7 @@
 import { m } from '$lib/paraglide/messages.js';
 import { SvelteMap } from 'svelte/reactivity';
 import TranscriptionBlock from './TranscriptionBlock.svelte';
-
-type TranscriptionBlockData = {
-	id: string;
-	annotationId: string;
-	documentId: string;
-	text: string;
-	label: string | null;
-	sortOrder: number;
-	version: number;
-};
+import type { TranscriptionBlockData } from '$lib/types';
 
 type SaveState = 'idle' | 'saving' | 'saved' | 'error';