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/icons.js

@@ -37,3 +37,13 @@ export const UPLOAD_ICON = `
   <path d="M12 15V4M8 8l4-4 4 4"/>
   <path d="M4 15v3a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2v-3"/>
 </svg>`;
+
+// Shown next to each file in the GPX tree, indicating whether it has
+// waypoints, a track, or both.
+export const WAYPOINT_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M12 21s7-7.58 7-12a7 7 0 0 0-14 0c0 4.42 7 12 7 12z"/>
+  <circle cx="12" cy="9" r="2.2"/>
+</svg>`;
+
+export const TRACK_ICON = GPX_ICON;