Show a GeoJSON icon next to files with a ready cache

GpxStore#tree now reports a "cached" flag per file (reusing the same
mtime freshness check geojson() already does); the file tree shows a
small curly-braces icon next to any file whose conversion is already
on disk, so it's visible which files will toggle on instantly versus
which still need to parse.

lib/maptool/gpx_store.rb

@@ -22,7 +22,10 @@ module Maptool
     end
 
     # Nested { type:, name:, path:, children: } hash describing every
-    # folder and .gpx/.fit file under the store root.
+    # folder and .gpx/.fit file under the store root. Each file also
+    # carries "cached" — whether its GeoJSON conversion is ready on disk
+    # (see `geojson` below) — so the tree UI can show a file's ready to
+    # view instantly, without a round trip just to find out.
     def tree
       { "type" => "folder", "name" => "root", "path" => "", "children" => list_children(@root, "") }
     end
@@ -112,7 +115,7 @@ module Maptool
       raise Errno::ENOENT, path unless File.file?(path)
 
       cache = geojson_cache_path(path)
-      if !File.file?(cache) || File.mtime(cache) < File.mtime(path)
+      unless geojson_cached?(path)
         data = path.downcase.end_with?(".fit") ? Fit.to_geojson(path) : Gpx.to_geojson(path)
         File.write(cache, JSON.generate(data))
       end
@@ -136,6 +139,11 @@ module Maptool
       "#{path}.geojson"
     end
 
+    def geojson_cached?(path)
+      cache = geojson_cache_path(path)
+      File.file?(cache) && File.mtime(cache) >= File.mtime(path)
+    end
+
     TRACK_EXTENSIONS = %w[.gpx .fit].freeze
 
     def safe_filename(filename)
@@ -165,9 +173,9 @@ 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, "kinds" => Gpx.kinds(full) }
+          { "type" => "file", "name" => entry, "path" => rel, "kinds" => Gpx.kinds(full), "cached" => geojson_cached?(full) }
         elsif entry.downcase.end_with?(".fit")
-          { "type" => "file", "name" => entry, "path" => rel, "kinds" => Fit.kinds(full) }
+          { "type" => "file", "name" => entry, "path" => rel, "kinds" => Fit.kinds(full), "cached" => geojson_cached?(full) }
         end
       end
     end