Convert GPX/FIT to GeoJSON at upload time, not first view

GpxStore#save now calls geojson() itself right after writing the
file, so the (potentially tens-of-seconds, for a large POI export)
conversion cost is paid during the upload — covered by the frontend's
existing "Uploading…" status — instead of on whichever checkbox
toggle happens to be first, which otherwise reads as a silent hang
indistinguishable from broken. An unparseable file still saves fine;
the error just surfaces on first view same as before.

lib/maptool/gpx_store.rb

@@ -30,12 +30,26 @@ module Maptool
     # Saves `io`'s contents as a .gpx/.fit file under `folder` (created if
     # needed), deriving a safe filename from `filename`. Returns the
     # stored file's path relative to the store root.
+    #
+    # Converts to GeoJSON and caches it right away (see `geojson` below)
+    # rather than waiting for the first time someone checks the file's box —
+    # a several-thousand-waypoint POI export is slow enough to parse that
+    # doing it lazily meant the *first* toggle silently hung (indistinguishable
+    # from broken), instead of the upload's own "Uploading…" status covering
+    # that wait like it already does for the upload itself. A file that fails
+    # to parse still saves fine — same as before, it just surfaces the error
+    # on first view instead of here.
     def save(folder:, filename:, io:)
       dir = resolve_dir(folder.to_s)
       FileUtils.mkdir_p(dir)
       name = unique_filename(dir, safe_filename(filename))
       File.binwrite(File.join(dir, name), io.read)
       relative = folder.to_s.empty? ? name : "#{folder}/#{name}"
+      begin
+        geojson(relative)
+      rescue Gpx::ParseError, Fit::ParseError
+        nil
+      end
       relative
     end