Spec and implement drag and drop polish #10

Open design questions for drag and drop:

• Visual feedback during drag: What does the drag preview look like? Just the filename, or icon + filename? What about multi-file drag?
• Drop indicators: Insertion line between items for reordering within a folder? Highlight on a folder to indicate "drop inside"? Auto-expand collapsed folder after hover delay?
• External drag out: Dragging files out of FileTree into another app (Finder, Mail, etc.) via file promise. Second-order feature — internal move is the priority.
• Drop validation edge cases: Dropping onto a file (reject or drop into parent?). Name conflicts at destination.
• Undo granularity: If multiple files are moved at once, is that one undo operation or many?