fix(fullscreen): complete drag-drop file move with auto-expand and ancestor guard

[KrisPowers]

Closes #74

Summary

• Adds a 600ms auto-expand timer: hovering a dragged item over a collapsed folder expands it after 600ms, matching standard OS drag-to-expand behavior
• Guards ancestor drops: dropping a folder onto one of its own descendants is now a silent no-op (the ancestor check in both onDragOver and onDrop uses startsWith(src + '/') to detect this)
• Fixes false dragLeave resets: onDragLeave now checks currentTarget.contains(relatedTarget) before clearing state, preventing flickering when the cursor moves over child spans within the same row

The underlying move via fs.rename IPC and the fe-node--dragover highlight were already wired; these changes complete the remaining acceptance criteria from the issue.

Test plan

• Drag a file onto a folder — file moves into the folder, tree refreshes
• Drag a folder onto another folder — entire directory moves, tree refreshes
• Drag a folder onto one of its own subfolders — no move, no error
• Drag a file onto itself or its own parent — no-op
• Hover a dragged item over a collapsed folder for >600ms — folder expands without dropping
• Release drag outside any folder — operation cancels, nothing changes