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).

views/index.erb

@@ -6,11 +6,15 @@
     <button type="button" class="gpx-panel__close" aria-label="Close">&times;</button>
   </div>
 
-  <form class="gpx-upload">
-    <input type="file" name="file" accept=".gpx" required>
-    <input type="text" name="folder" placeholder="folder/subfolder (optional)">
-    <button type="submit">Upload</button>
-    <p class="gpx-upload__status" hidden></p>
+  <div class="gpx-dropzone">
+    Drop .gpx files here, or click to browse
+    <input type="file" class="gpx-dropzone__input" accept=".gpx" multiple hidden>
+  </div>
+  <p class="gpx-upload__status" hidden></p>
+
+  <form class="gpx-new-folder">
+    <input type="text" name="path" placeholder="new folder path (e.g. Hiking/2026)">
+    <button type="submit">+ folder</button>
   </form>
 
   <div class="gpx-tree"></div>