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/js/gpx.js

@@ -1,5 +1,5 @@
 import { Popup } from "/vendor/maplibre-gl/maplibre-gl.mjs";
-import { GPX_ICON, NEW_FOLDER_ICON, UPLOAD_ICON } from "/js/icons.js";
+import { GPX_ICON, NEW_FOLDER_ICON, UPLOAD_ICON, WAYPOINT_ICON, TRACK_ICON } from "/js/icons.js";
 import { notifySidePanelOpen, onlyOneSidePanelOpen } from "/js/panels.js";
 
 const TREE_URL = "/api/gpx/tree";
@@ -205,9 +205,11 @@ export class GpxPanelControl {
     checkbox.checked = this._visible.has(file.path);
     checkbox.addEventListener("change", () => this._toggleFile(file.path, checkbox.checked));
     const name = document.createElement("span");
+    name.className = "gpx-tree__file-name";
     name.textContent = file.name;
     name.title = file.name;
     label.appendChild(checkbox);
+    (file.kinds || []).forEach((kind) => label.appendChild(this._kindIcon(kind)));
     label.appendChild(name);
     row.appendChild(label);
     row.appendChild(this._deleteButton(() => this._deleteFile(file.path)));
@@ -215,6 +217,19 @@ export class GpxPanelControl {
     return row;
   }
 
+  _kindIcon(kind) {
+    const icon = document.createElement("span");
+    icon.className = "gpx-tree__kind-icon";
+    if (kind === "waypoint") {
+      icon.innerHTML = WAYPOINT_ICON;
+      icon.title = "Has waypoints";
+    } else {
+      icon.innerHTML = TRACK_ICON;
+      icon.title = "Has a track";
+    }
+    return icon;
+  }
+
   _deleteButton(onClick) {
     const button = document.createElement("button");
     button.type = "button";