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 {

public/js/gpx.js

@@ -271,10 +271,23 @@ export class GpxPanelControl {
     checkbox.type = "checkbox";
     checkbox.checked = this._visible.has(file.path);
     checkbox.addEventListener("change", () => this._toggleFile(file.path, checkbox.checked, checkbox));
-    const name = document.createElement("span");
+    // Once a .gpx/.fit is cached, its converted GeoJSON is the thing that's
+    // actually shown on the map from here on (see GpxStore#geojson) — the
+    // name reflects that (matching the real "<path>.geojson" cache file on
+    // disk) rather than still reading as a plain, un-converted .gpx/.fit.
+    // A direct .geojson upload already ends in .geojson, so it's left
+    // alone. It's an <a>, not a <span>: clicking it downloads that GeoJSON
+    // (?download=1) — the only way to get it out of the app, since the
+    // original .gpx/.fit is never deleted but also never re-shown once
+    // converted. An <a> nested in a <label> doesn't also toggle the
+    // checkbox on click (unlike a plain element), so this doesn't fight
+    // the row's own toggle-on-map behavior.
+    const displayName = file.cached && !file.name.toLowerCase().endsWith(".geojson") ? `${file.name}.geojson` : file.name;
+    const name = document.createElement("a");
     name.className = "gpx-tree__file-name";
-    name.textContent = file.name;
-    name.title = file.name;
+    name.href = `${gpxFileUrl(file.path)}?download=1`;
+    name.textContent = displayName;
+    name.title = `Download ${displayName}`;
     label.appendChild(checkbox);
     (file.kinds || []).forEach((kind) => label.appendChild(this._kindIcon(kind)));
     if (file.cached) label.appendChild(this._cachedIcon());