Consolidate original/GeoJSON download into one menu button

The file name doubling as a GeoJSON-only download link was easy to
miss as a link at all, and only ever covered half of what's actually
downloadable. One button next to delete now opens a small menu
(Original file / GeoJSON) instead -- costs a click when downloading
several files in a row, but both formats are reachable from one,
discoverable place. Only one menu open at a time, closed on any
outside click.

Verified in a real browser: opens with both items, closes on outside
click, no console errors.

public/css/style.css

@@ -379,11 +379,20 @@ html, body {
   height: 14px;
 }
 
+.gpx-tree__download-wrapper {
+  position: relative;
+  flex-shrink: 0;
+  display: flex;
+}
+
 .gpx-tree__download {
   flex-shrink: 0;
   display: flex;
+  border: none;
+  background: none;
   color: #999;
   padding: 0 4px;
+  cursor: pointer;
 }
 
 .gpx-tree__download svg {
@@ -395,6 +404,38 @@ html, body {
   color: #333;
 }
 
+.gpx-tree__download-menu {
+  position: absolute;
+  top: 100%;
+  right: 0;
+  z-index: 20;
+  min-width: max-content;
+  background: #fff;
+  border: 1px solid #ddd;
+  border-radius: 6px;
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.15);
+  padding: 4px;
+  font-size: 13px;
+}
+
+.gpx-tree__download-menu[hidden] {
+  display: none;
+}
+
+.gpx-tree__download-menu-item {
+  display: block;
+  padding: 6px 10px;
+  border-radius: 4px;
+  color: inherit;
+  text-decoration: none;
+  white-space: nowrap;
+  cursor: pointer;
+}
+
+.gpx-tree__download-menu-item:hover {
+  background: #f0f0f0;
+}
+
 .gpx-tree__delete {
   flex-shrink: 0;
   border: none;