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.

lib/maptool/gpx.rb

@@ -8,6 +8,19 @@ module Maptool
   module Gpx
     class ParseError < StandardError; end
 
+    # Cheap content-type check for the file tree's icons: a plain substring
+    # search rather than a full REXML parse, since some uploaded files (e.g.
+    # a several-thousand-waypoint POI export) are large enough that parsing
+    # the whole DOM just to answer "does it have any of these tags" would
+    # make every tree fetch noticeably slower.
+    def self.kinds(path)
+      content = File.read(path)
+      kinds = []
+      kinds << "waypoint" if content.include?("<wpt")
+      kinds << "track" if content.include?("<trk")
+      kinds
+    end
+
     def self.to_geojson(path)
       root = REXML::Document.new(File.read(path)).root
       raise ParseError, "not a GPX file" unless root && root.name == "gpx"

lib/maptool/gpx_store.rb

@@ -1,4 +1,5 @@
 require "fileutils"
+require_relative "gpx"
 
 module Maptool
   # Filesystem-backed store for uploaded GPX files, organized in
@@ -117,7 +118,7 @@ module Maptool
         if File.directory?(full)
           { "type" => "folder", "name" => entry, "path" => rel, "children" => list_children(full, rel) }
         elsif entry.downcase.end_with?(".gpx")
-          { "type" => "file", "name" => entry, "path" => rel }
+          { "type" => "file", "name" => entry, "path" => rel, "kinds" => Gpx.kinds(full) }
         end
       end
     end