Let a cached file's GeoJSON be downloaded by clicking its tree name

Once a .gpx/.fit is cached, the tree shows it under its converted name
("ride.gpx" -> "ride.gpx.geojson", matching the real cache file next
to it on disk) instead of still reading as un-converted -- the
GeoJSON is what's actually on the map from that point on, and the
original is never deleted, just not re-shown as itself. The name is
now a real link (GET /api/gpx/file/<path>?download=1) instead of
plain text, the only way to get that converted form out of the app.

Verified in a real browser that an <a> nested in a <label> claims its
own click rather than the label forwarding it to the checkbox, so
downloading and toggling visibility don't fight over the same click.

docs/architecture.adoc

@@ -285,7 +285,23 @@ 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>`
+waiting on.
++
+Once a `.gpx`/`.fit` is cached, the tree shows it under its converted
+name instead — `"ride.gpx"` reads as `"ride.gpx.geojson"`, matching
+the real cache file sitting next to it on disk — since the GeoJSON,
+not the original, is what's actually rendered on the map from that
+point on. The original is never deleted or re-parsed once cached, just
+not re-shown as itself; the tree's file name is a real link
+(`GET /api/gpx/file/<path>?download=1`), the only way to get that
+converted form out of the app as an actual file, since the cache is
+otherwise an implementation detail with no name of its own to expose.
+It's an `<a>`, not a `<span>`, specifically so clicking it downloads
+instead of also toggling the row's checkbox — a native `<a>`/`<button>`
+nested inside a `<label>` claims its own click instead of the label
+forwarding it to the associated control, so the two behaviors don't
+fight over the same click. A directly-uploaded `.geojson` keeps its
+own name (nothing to convert) and downloads unchanged. `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>`