Surface GPX/FIT parse errors instead of silently rendering nothing

_addLayers handed MapLibre a URL and let it fetch the GeoJSON
internally — a 422 (a file REXML/Fit can't parse) just meant the
layer never appeared, with nothing telling the user why. It now
fetches the file itself first, alerts with the actual error and
un-checks the box on failure.

docs/architecture.adoc

@@ -189,7 +189,13 @@ equivalent, so it never produces `Point` features. MapLibre's
 `geometry-type` expression treats `MultiLineString` the same as
 `LineString`, so a single GeoJSON source per file with a `line` layer
 and a `circle` layer (each filtered by geometry type) is enough to
-render both.
+render both. Checking a file fetches its GeoJSON itself
+(`public/js/gpx.js`'s `_addLayers`) and hands the parsed result to
+`addSource` — rather than passing MapLibre a URL and letting it fetch
+internally — specifically so a `422` (a file REXML or `Fit` can't
+parse) surfaces as an alert and un-checks the box instead of the layer
+just silently never appearing, which is what a `data: url` source does
+on a failed fetch.
 
 FIT parsing:: No gem was added for this — the one Ruby option
 (`fit-parser`, tjwallace/fit) hasn't been touched since 2022 and pulls