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.

public/css/style.css

@@ -360,6 +360,12 @@ html, body {
   overflow: hidden;
   text-overflow: ellipsis;
   white-space: nowrap;
+  color: inherit;
+  text-decoration: none;
+}
+
+.gpx-tree__file-name:hover {
+  text-decoration: underline;
 }
 
 .gpx-tree__kind-icon {