Add support for dragging files from the OS into the file explorer sidebar.
Files are copied (not moved) into the target directory within the worktree.
- Preload detects native file drops via capture-phase listener and resolves
the drop target (editor, terminal, or file-explorer) from DOM attributes
- New IPC handler `fs:importExternalPaths` copies files/directories with
symlink pre-scanning, path-traversal protection, and dedup on conflict
- FileExplorer tracks native drag state separately from internal drag state
to show correct drop highlights without interfering with tree reordering
- FileExplorerRow expands directories on hover during native drags
- useFileExplorerImport hook subscribes to preload IPC events and drives
the import → refresh → reveal pipeline
Includes review fixes:
- Clear nativeDropTargetDir on row drag-leave to prevent stale highlights
- Clear native drag state on early return in useFileExplorerImport
- Extract row drag logic to useFileExplorerRowDrag hook
- Split import tests into dedicated filesystem-import.test.ts
- Use T[] syntax instead of Array<T> per lint rules
- Use ternary expressions for simple if/else per lint rules
