Users can now drag files and folders from their file system directly
onto folders in the workspace tree to trigger an upload to that location.

Changes:
- Add dropZoneUtils.ts with shared utilities for handling file drops
- Update TreeBrowser to detect and handle file system drops vs internal moves
- Update UploadFiles to accept pre-populated files via droppedFiles prop
- Wire up Workspaces component to pass dropped files to upload modal

Cross-platform compatible using the File System Access API (webkitGetAsEntry)
which is supported by all major browsers.

- Tighten file detection: exclude drags containing application/json
  to prevent internal tree drags being misidentified as file drops
- Add .catch() to readFilesFromDragEvent in TreeBrowser to handle
  errors (permissions, broken symlinks) instead of swallowing them
- Fix collapsed folder drop target: search workspace tree recursively
  instead of only expandedNodes, so drops onto collapsed folders go
  to the correct folder rather than silently falling back to root
- Remove unused readingDroppedFiles state from TreeBrowser
- Remove unused dragEventContainsInternalData export from dropZoneUtils

…d selections

Collect all FileSystemEntry references synchronously from the DataTransfer
before any async work. The browser invalidates the DataTransfer after the
synchronous event handler returns, so awaiting inside the iteration loop
caused items beyond the first to be silently lost.

Also removes redundant double-call to webkitGetAsEntry() per item.

Reject mixed selections (folders + files, or multiple folders) which
caused duplicate entries due to how the OS includes both a folder and
its visible children as separate DataTransfer items.

Now only two shapes are accepted:
 1. A single directory (full hierarchy preserved)
 2. One or more files with no directories

Show an alert to the user when an unsupported selection is dropped.