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.

public/js/gpx.js

@@ -6,6 +6,7 @@ import {
   WAYPOINT_ICON,
   TRACK_ICON,
   ROUTE_ICON,
+  GEOJSON_ICON,
   MAP_MARKER_ICON,
   POI_ICONS,
   PARK4NIGHT_TAGS,
@@ -276,6 +277,7 @@ export class GpxPanelControl {
     name.title = file.name;
     label.appendChild(checkbox);
     (file.kinds || []).forEach((kind) => label.appendChild(this._kindIcon(kind)));
+    if (file.cached) label.appendChild(this._cachedIcon());
     label.appendChild(name);
     row.appendChild(label);
     row.appendChild(this._deleteButton(() => this._deleteFile(file.path)));
@@ -299,6 +301,14 @@ export class GpxPanelControl {
     return icon;
   }
 
+  _cachedIcon() {
+    const icon = document.createElement("span");
+    icon.className = "gpx-tree__kind-icon";
+    icon.innerHTML = GEOJSON_ICON;
+    icon.title = "Ready to view instantly (GeoJSON cached)";
+    return icon;
+  }
+
   _deleteButton(onClick) {
     const button = document.createElement("button");
     button.type = "button";