fix: preserve explorer drag overlay sizing

[hhhjin]

Summary

• preserve explorer drag overlay width and padding from the dragged row
• pass the source DOM element into the explorer overlay renderer
• add tests for the extracted overlay style helper

Validation

• review gate: no actionable findings
• pnpm lint:fix
• pnpm ts:check:desktop
• pnpm -C apps/desktop test -- src/components/dnd/explorer-drag-overlay.test.tsx

[gemini-code-assist]

Code Review

This pull request introduces dynamic styling for the explorer drag overlay by capturing and applying the dimensions and padding of the source element. It adds a sourceElement prop to the ExplorerDragOverlay component, implements a utility function getExplorerDragOverlayStyle to extract styles using getComputedStyle and getBoundingClientRect, and includes corresponding unit tests. I have no feedback to provide as there were no review comments to evaluate.