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";

public/js/icons.js

@@ -54,6 +54,16 @@ export const WAYPOINT_ICON = `
 
 export const TRACK_ICON = GPX_ICON;
 
+// Shown next to a file whose GeoJSON conversion is already cached on disk
+// (GpxStore#geojson) — ready to view instantly instead of paying REXML's
+// parse cost on click. Curly braces read as "structured data ready", the
+// same shorthand most editors/tools use for JSON.
+export const GEOJSON_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M9 4c-1.5 0-2 .8-2 2v3c0 1-.5 1.5-1.5 1.5 1 0 1.5.5 1.5 1.5v3c0 1.2.5 2 2 2"/>
+  <path d="M15 4c1.5 0 2 .8 2 2v3c0 1 .5 1.5 1.5 1.5-1 0-1.5.5-1.5 1.5v3c0 1.2-.5 2-2 2"/>
+</svg>`;
+
 // Same shape as TRACK_ICON, dashed — a route is planned rather than
 // recorded, the same visual convention most map apps use for the
 // distinction.