Add GPX drag & drop, folder management, delete, and folder-first sort

Uploading is now drag & drop (or click to browse) instead of a form;
folders can be created any time, not just during upload; files can be
dragged onto a folder to move them, or dropped elsewhere in the tree
to move back to the top level; files and folders can be deleted. Both
drag sources (an OS file drag and dragging an existing tree entry)
land on the same drop targets, told apart by a custom drag payload
type versus the browser's own "Files" type.

Also sort folders before files in the tree (previously interleaved
alphabetically).

lib/maptool/gpx_store.rb

@@ -41,6 +41,42 @@ module Maptool
       path
     end
 
+    def create_folder(path)
+      raise InvalidPath if path.to_s.strip.empty?
+
+      FileUtils.mkdir_p(resolve_dir(path.to_s))
+    end
+
+    def delete_file(relative_path)
+      path = resolve_file(relative_path)
+      raise Errno::ENOENT, path unless File.file?(path)
+
+      File.delete(path)
+    end
+
+    def delete_folder(relative_path)
+      raise InvalidPath if relative_path.to_s.strip.empty?
+
+      dir = resolve_dir(relative_path.to_s)
+      raise InvalidPath if dir == @root
+      raise Errno::ENOENT, dir unless File.directory?(dir)
+
+      FileUtils.rm_rf(dir)
+    end
+
+    # Moves a file into `to_folder` (created if needed), disambiguating the
+    # filename on collision. Returns the new store-relative path.
+    def move(from:, to_folder:)
+      source = resolve_file(from)
+      raise Errno::ENOENT, source unless File.file?(source)
+
+      dest_dir = resolve_dir(to_folder.to_s)
+      FileUtils.mkdir_p(dest_dir)
+      name = unique_filename(dest_dir, File.basename(source))
+      FileUtils.mv(source, File.join(dest_dir, name))
+      to_folder.to_s.empty? ? name : "#{to_folder}/#{name}"
+    end
+
     private
 
     def resolve_dir(folder)
@@ -72,7 +108,9 @@ module Maptool
     end
 
     def list_children(dir, relative)
-      Dir.children(dir).sort.filter_map do |entry|
+      entries = Dir.children(dir).sort_by { |entry| [File.directory?(File.join(dir, entry)) ? 0 : 1, entry] }
+
+      entries.filter_map do |entry|
         full = File.join(dir, entry)
         rel = relative.empty? ? entry : "#{relative}/#{entry}"