Cache parsed GPX/FIT GeoJSON to fix large POI-export loads

A several-thousand-waypoint park4night area export re-parses the
whole file with REXML on every toggle — ~2.4s for a 5k-waypoint/3.6MB
sample, so tens of seconds for a 50k-waypoint one, likely the actual
cause behind a "Could not load" report on a 41.5MB export. MapLibre
already clusters the waypoint layer client-side, so the bottleneck was
never rendering — cache the GeoJSON conversion alongside the source
file (invalidated by mtime) instead of a bigger bbox/tiling rework.

lib/maptool/gpx_store.rb

@@ -1,4 +1,5 @@
 require "fileutils"
+require "json"
 require_relative "gpx"
 require_relative "fit"
 
@@ -58,6 +59,7 @@ module Maptool
       raise Errno::ENOENT, path unless File.file?(path)
 
       File.delete(path)
+      File.delete(geojson_cache_path(path)) if File.file?(geojson_cache_path(path))
     end
 
     def delete_folder(relative_path)
@@ -79,10 +81,30 @@ module Maptool
       dest_dir = resolve_dir(to_folder.to_s)
       FileUtils.mkdir_p(dest_dir)
       name = unique_filename(dest_dir, File.basename(source))
-      FileUtils.mv(source, File.join(dest_dir, name))
+      dest = File.join(dest_dir, name)
+      FileUtils.mv(source, dest)
+      FileUtils.mv(geojson_cache_path(source), geojson_cache_path(dest)) if File.file?(geojson_cache_path(source))
       to_folder.to_s.empty? ? name : "#{to_folder}/#{name}"
     end
 
+    # The GeoJSON conversion (Gpx.to_geojson/Fit.to_geojson) re-parses the
+    # whole file with REXML every call — fine for a small track, but a
+    # several-thousand-waypoint POI export (e.g. a park4night area export)
+    # can take tens of seconds, every single time the file is toggled
+    # visible. Cached alongside the source file and invalidated by mtime,
+    # so only the first read after a save (or edit) pays that cost.
+    def geojson(relative_path)
+      path = resolve_file(relative_path)
+      raise Errno::ENOENT, path unless File.file?(path)
+
+      cache = geojson_cache_path(path)
+      if !File.file?(cache) || File.mtime(cache) < File.mtime(path)
+        data = path.downcase.end_with?(".fit") ? Fit.to_geojson(path) : Gpx.to_geojson(path)
+        File.write(cache, JSON.generate(data))
+      end
+      File.read(cache)
+    end
+
     private
 
     def resolve_dir(folder)
@@ -96,6 +118,10 @@ module Maptool
       path == @root || path.start_with?(@root + File::SEPARATOR)
     end
 
+    def geojson_cache_path(path)
+      "#{path}.geojson"
+    end
+
     TRACK_EXTENSIONS = %w[.gpx .fit].freeze
 
     def safe_filename(filename)