test: simplify useNodeDrag test to use real stores instead of manual mock state

## Problem

`useNodeDrag.test.ts` has a massive `vi.hoisted()` block that manually mocks every dependency (`canvasStore`, `layoutStore`, `layoutMutations`, `useNodeSnap`, etc.) with granular fake state. This makes the test fragile, hard to maintain, and mostly validates the mock plumbing rather than real behavior.

## Proposed Solution

- Use `createTestingPinia({ stubActions: false })` for Pinia stores (`canvasStore`, `settingStore`)
- Use the real `layoutStore` singleton (Yjs-backed, not Pinia) seeded via `createNode()`
- Use real `useLayoutMutations()` (thin wrapper over `layoutStore`)
- Use real `useNodeSnap()` (reads from `settingStore`)
- Keep only edge mocks: `AutoPanController`, `useShiftKeySync`, `useTransformState`, `requestAnimationFrame`

## Notes

- `canvasStore` calls `useAppMode()` at definition time, which needs a mock
- `layoutStore` is a class singleton, not Pinia — reset via creating a fresh instance or clearing state in `beforeEach`
- `useNodeDrag` is wrapped in `createSharedComposable` — keep the existing mock for that or use `effectScope()`
- Estimated effort: M (1-3h)

## Files

- `src/renderer/extensions/vueNodes/layout/useNodeDrag.test.ts`
- `src/renderer/extensions/vueNodes/layout/useNodeDrag.ts`

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10765-test-simplify-useNodeDrag-test-to-use-real-stores-instead-of-manual-mock-state-3346d73d365081a68fe2deb4169185f0) by [Unito](https://www.unito.io)

Start with src/renderer/extensions/vueNodes/layout/useNodeDrag.test.ts and compare it with src/renderer/extensions/vueNodes/layout/useNodeDrag.ts. Run the focused useNodeDrag tests, then replace the manual dependency state with the real stores and retain only the listed edge mocks. Done means the tests pass while exercising real store, mutation, and snapping behavior.

Written by the indexing model from the issue text.