Badge the exception (not yet cached) instead of the near-universal case

GpxStore#save converts eagerly, so "cached" is true for nearly every
file in the tree already — badging that was just noise. Flipped to
PENDING_ICON, shown only for the rare case: a .gpx/.fit dropped
straight into data/gpx/ on the filesystem, bypassing the app, and not
yet opened.

Also add an explicit download button next to delete (DOWNLOAD_ICON) —
the file name itself has been a download link since the last commit,
but isn't obviously clickable at a glance.

docs/architecture.adoc

@@ -282,26 +282,31 @@ 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.
 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.
+returns it directly, no cache file, no `Gpx`/`Fit` involved.
+`GpxStore#save` converting eagerly means `cached` is `true` for
+almost everything in the tree — the one exception is a `.gpx`/`.fit`
+dropped straight into `data/gpx/` on the filesystem, bypassing the
+app, and not yet opened. Badging the common case would just be noise,
+so the file tree's icon (`PENDING_ICON`, `public/js/icons.js`) marks
+that exception instead of the (near-universal) ready state.
 +
 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>`
+not re-shown as itself. Two ways to get that converted form out of the
+app as an actual file (`GET /api/gpx/file/<path>?download=1`) — the
+cache is otherwise an implementation detail with no name of its own to
+expose: the file name itself is a real link (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), and a dedicated download button sits next to delete for anyone
+who wouldn't guess the name itself is clickable. A directly-uploaded
+`.geojson` keeps its own name (nothing to convert) and downloads
+unchanged either way. `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>`