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.

docs/architecture.adoc

@@ -221,11 +221,11 @@ This is a live, online-only overlay — it always needs internet access
 and is unrelated to the offline PMTiles basemap above. Per RainViewer's
 terms, the API is free for personal/non-commercial use.
 
-== GPX/FIT tracks and waypoints
+== GPX/FIT/GeoJSON tracks and waypoints
 
 The "Tracks" map control (`public/js/gpx.js`) opens a side panel to
-upload GPX or FIT files by dragging them in, organize them into
-folders (also by dragging), and toggle each one on the map
+upload GPX, FIT or GeoJSON files by dragging them in, organize them
+into folders (also by dragging), and toggle each one on the map
 individually.
 
 Storage:: Files are stored as-is under `data/gpx/`, in a
@@ -243,13 +243,18 @@ which the panel renders recursively (folders as `<details>`, files as
 a checkbox, both with a delete button).
 
 Drag & drop:: The dropzone at the top of the panel accepts one or more
-`.gpx`/`.fit` files, or a `.zip` (`POST /api/gpx`, multipart) into the
-top level, or click it to browse instead. A `.zip` — Garmin's bulk/FIT
-export format — isn't stored itself: `save_zip_entries` (`app.rb`)
-expands it with `rubyzip` and saves each `.gpx`/`.fit` entry it
-contains individually (flattened — any folder structure inside the
-zip is discarded), skipping anything else (readme files, etc.) it
-might hold. An already-uploaded file (a
+`.gpx`/`.fit`/`.geojson` files, or a `.zip` (`POST /api/gpx`,
+multipart) into the top level, or click it to browse instead. A
+`.geojson` file needs no format conversion — it's already the shape
+`gpx.js` expects — so `GpxStore#geojson` just serves it straight from
+disk (see "Rendering" below); useful for a file already in this app's
+own schema, e.g. one exported elsewhere in that shape, or a previous
+`GET /api/gpx/file` response saved and re-uploaded. A `.zip` —
+Garmin's bulk/FIT export format — isn't stored itself:
+`save_zip_entries` (`app.rb`) expands it with `rubyzip` and saves each
+`.gpx`/`.fit`/`.geojson` entry it contains individually (flattened —
+any folder structure inside the zip is discarded), skipping anything
+else (readme files, etc.) it might hold. An already-uploaded file (a
 `.gpx-tree__file` row, `draggable`) can be dragged onto a folder to
 move it there (`POST /api/gpx/move`), or dropped anywhere else in the
 tree to move it back to the top level; an OS file can also be dropped
@@ -276,7 +281,11 @@ status) instead of on whichever toggle happens to be first — a silent
 multi-second hang on a checkbox click reads as broken in a way the
 same wait during an upload doesn't. A file that fails to parse still
 saves fine either way; the error just surfaces on first view instead.
-`Gpx` parses the GPX with REXML: `<wpt>`
+A `.geojson` upload skips this path entirely — `geojson` reads and
+returns it directly, no cache file, no `Gpx`/`Fit` involved; the file
+tree's cached-file icon (`GEOJSON_ICON`, `public/js/icons.js`) always
+shows for one, since there's no conversion step it could ever be
+waiting on. `Gpx` parses the GPX with REXML: `<wpt>`
 waypoints become `Point` features (with `name`, `desc` and `time`
 carried through as properties), each `<trk>` becomes one
 `MultiLineString` feature (one line per `<trkseg>`), and each `<rte>`