Support uploading .geojson files directly

A .geojson file needs no GPX/FIT conversion — it's already the shape
gpx.js expects — so GpxStore#geojson now just reads and returns it
as-is, no cache sidecar. save/tree/the upload dropzone and its zip
handling all pick it up through the existing TRACK_EXTENSIONS list.

Also fixes a bug the new "cached" tree icon would have shipped with:
a GPX/FIT's own .geojson cache sidecar (e.g. "ride.gpx.geojson") also
matches the new .geojson branch in list_children, which would have
listed it as a second, phantom tree entry — excluded explicitly, with
a regression test.

public/js/gpx.js

@@ -620,9 +620,12 @@ export class GpxPanelControl {
 
   async _uploadFiles(fileList, folder) {
     // .zip: Garmin's bulk/FIT export format — the server expands it into
-    // its .gpx/.fit entries, so it's accepted here even though the tree
-    // never shows a .zip itself.
-    const files = [...fileList].filter((file) => /\.(gpx|fit|zip)$/i.test(file.name));
+    // its .gpx/.fit/.geojson entries, so it's accepted here even though the
+    // tree never shows a .zip itself. .geojson needs no server-side
+    // conversion (GpxStore#geojson serves it as-is) — accepted here so a
+    // file already in this app's own shape (e.g. a previous /api/gpx/file
+    // response, saved and re-uploaded) can skip GPX/FIT entirely.
+    const files = [...fileList].filter((file) => /\.(gpx|fit|geojson|zip)$/i.test(file.name));
     if (files.length === 0) return;
 
     this._setStatus(files.length === 1 ? "Uploading…" : `Uploading ${files.length} files…`);