refactor(dragdrop): generalize createBlockDragDrop<T extends { id: string }>

Removes the hard-typed TranscriptionBlockData constraint so JourneyEditor
can reuse the pointer-drag module without importing transcription types.
Selector contract (data-block-wrapper / data-drag-handle) unchanged.
Adds type-regression guard test verified via tsc --noEmit.

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

frontend/src/lib/document/transcription/useBlockDragDrop.svelte.test.ts

@@ -1,7 +1,33 @@
-import { describe, it, expect, vi } from 'vitest';
+import { describe, it, expect, vi, expectTypeOf } from 'vitest';
 import { createBlockDragDrop } from './useBlockDragDrop.svelte';
 import type { TranscriptionBlockData } from '$lib/shared/types';
 
+// ---------------------------------------------------------------------------
+// Type-regression guard: createBlockDragDrop must accept any T extends {id: string}
+// so JourneyEditor can reuse it without importing TranscriptionBlockData.
+// This test fails with "Expected 0 type arguments, but got 1" via tsc --noEmit
+// until the function is made generic.
+// ---------------------------------------------------------------------------
+describe('createBlockDragDrop — generic type guard', () => {
+	it('accepts items shaped as { id: string; position: number } — not only TranscriptionBlockData', () => {
+		type SimpleItem = { id: string; position: number };
+		const items: SimpleItem[] = [
+			{ id: 'item-1', position: 0 },
+			{ id: 'item-2', position: 1 }
+		];
+		const onReorder = vi.fn();
+		const dd = createBlockDragDrop<SimpleItem>({ getSortedBlocks: () => items, onReorder });
+		// Verify the hook is functional with the new type — state reads must work
+		expect(dd.draggedBlockId).toBeNull();
+		expect(dd.dragOffsetY).toBe(0);
+	});
+
+	it('TranscriptionBlockData caller still compiles — regression guard for existing transcription editor', () => {
+		// If the generic constraint is wrong this line fails tsc --noEmit
+		expectTypeOf(createBlockDragDrop<TranscriptionBlockData>).toBeFunction();
+	});
+});
+
 function makeBlock(id: string, sortOrder: number): TranscriptionBlockData {
 	return {
 		id,

frontend/src/lib/document/transcription/useBlockDragDrop.svelte.ts

@@ -1,11 +1,12 @@
-import type { TranscriptionBlockData } from '$lib/shared/types';
+type Options<T extends { id: string }> = {
-
+	getSortedBlocks: () => T[];
-type Options = {
-	getSortedBlocks: () => TranscriptionBlockData[];
 	onReorder: (blockIds: string[]) => void;
 };
 
-export function createBlockDragDrop({ getSortedBlocks, onReorder }: Options) {
+export function createBlockDragDrop<T extends { id: string }>({
+	getSortedBlocks,
+	onReorder
+}: Options<T>) {
 	let draggedBlockId = $state<string | null>(null);
 	let dropTargetIdx = $state<number | null>(null);
 	let dragOffsetY = $state(0);