Add file-kind icons to the GPX tree, fix drop into an empty panel

Each tree entry now shows a small waypoint/track icon based on a
cheap substring check for <wpt>/<trk> (Maptool::Gpx.kinds) rather than
a full REXML parse, since re-parsing a several-thousand-waypoint file
on every tree fetch would make the panel noticeably slower to open.

Also fix a real drop-target bug: .gpx-tree had no flex:1, so a
short/empty tree left blank panel space below it that wasn't part of
the drag & drop target — dropping a file there silently did nothing.

public/css/style.css

@@ -292,6 +292,11 @@ html, body {
 }
 
 .gpx-tree {
+  /* flex:1 (not just overflow-y) so the tree fills the rest of the panel
+     even when short/empty — otherwise the blank space below it wouldn't
+     be part of the drop target and dropping a file there would silently
+     do nothing. */
+  flex: 1;
   overflow-y: auto;
   padding: 8px 12px;
 }
@@ -342,12 +347,23 @@ html, body {
   cursor: pointer;
 }
 
-.gpx-tree__file label span {
+.gpx-tree__file-name {
   overflow: hidden;
   text-overflow: ellipsis;
   white-space: nowrap;
 }
 
+.gpx-tree__kind-icon {
+  flex-shrink: 0;
+  display: flex;
+  color: #999;
+}
+
+.gpx-tree__kind-icon svg {
+  width: 14px;
+  height: 14px;
+}
+
 .gpx-tree__delete {
   flex-shrink: 0;
   border: none;