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/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.