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

internal/api/router.go

@@ -43,6 +43,8 @@ func NewRouter(db *sql.DB, dataDir, secret string) http.Handler {
 	mux.HandleFunc("POST /api/files/trash/empty", s.requireAuth(s.handleEmptyTrash))
 	mux.HandleFunc("POST /api/files/search", s.requireAuth(s.handleSearchFiles))
 	mux.HandleFunc("POST /api/files/shared", s.requireAuth(s.handleListSharedFiles))
+	mux.HandleFunc("POST /api/files/upload-image", s.requireAuth(s.handleUploadImage))
+	mux.HandleFunc("GET /api/files/image/", s.requireAuth(s.handleServeImage))
 
 	// Sharing
 	mux.HandleFunc("POST /api/share", s.requireAuth(s.handleShareFile))