perf(editor): apply useRef pattern to useUnifiedDragDrop callbacks for stable references

## Summary

The `useUnifiedDragDrop` hook in `resume-builder-ui/src/hooks/editor/useUnifiedDragDrop.ts` includes `sections` in the dependency array of `handleDragEnd` (line 381), which causes the callback to be recreated every time the sections array changes. This can be avoided by using the same `useRef` pattern already applied in
`useSectionManagement`.

## Context

In a prior PR (#244), we optimized `useSectionManagement` by storing `sections` in a `useRef` and reading from `ref.current` inside `useCallback`, allowing callbacks to remain stable across re-renders. The same pattern was not applied to `useUnifiedDragDrop`.

While the performance impact is low (drag handlers fire infrequently), applying the pattern here improves consistency and prevents `DndContext` from receiving new function props on every sections change.

## What to change

**File:** `resume-builder-ui/src/hooks/editor/useUnifiedDragDrop.ts`

1. Add a `sectionsRef` using `useRef` near the top of the hook:
```ts
const sectionsRef = useRef(sections);
sectionsRef.current = sections;

2. Replace direct sections usage inside handleDragStart and handleDragEnd with sectionsRef.current.
3. Remove sections from the dependency array of handleDragEnd:
// Before
[setSections, sections]

// After
[setSections]

Reference implementation

See useSectionManagement.ts for the existing pattern to follow.

Acceptance criteria

- sectionsRef is added and kept in sync
- handleDragStart and handleDragEnd use sectionsRef.current instead of sections
- sections is removed from useCallback dependency arrays
- Drag-and-drop for sections, items, and subitems still works correctly
- Existing tests pass (npm test)