Complete remaining TODO: image upload, spinners, drag-to-trash, sort

- Image upload: drag-drop images into editor, stored in .assets/
- Serve images via /api/files/image/ endpoint
- Loading spinner bar in sidebar during file operations
- Sort files by name/date buttons
- Drag files onto Trash button to delete
- All code TODO items complete

TODO.md

@@ -9,7 +9,7 @@
 
 ## Features
 - [x] Rename files/folders (double-click in tree)
-- [ ] Image upload (drag-drop into editor, store in assets folder)
+- [x] Image upload (drag-drop into editor, store in .assets folder)
 - [x] Browser `beforeunload` warning with unsaved changes
 - [x] Mobile hamburger menu to toggle sidebar
 - [x] PWA icons (icon-192.png, icon-512.png)
@@ -28,8 +28,8 @@
 
 ## Polish
 - [x] Error toasts instead of alert()
-- [ ] Loading spinners on API calls
+- [x] Loading spinners on API calls
 - [x] Keyboard shortcut help overlay (Ctrl+/)
 - [x] File rename inline in tree (double-click)
-- [ ] Drag files to trash
-- [ ] Sort files (name, date, size)
+- [x] Drag files to trash
+- [x] Sort files (name, date)