Add an Alt+click debug inspector, NPMap4-infobox style

NPMap4 has infoboxControl: a generic "click anything, show every raw
property" debug view, kept separate from a dataset's own curated popup.
Not portable code (Leaflet), but the concept is: Alt+click anywhere now
opens a popup listing every rendered feature at that point via
map.queryRenderedFeatures — base map OSM tags included, not just this
app's own GPX/FIT/radar layers — with a raw key/value table per feature.

No dedicated UI for it on purpose: a keyboard-modified click needs no
button, toggle state, or cursor change of its own, and it's fine for it to
coexist with a layer's own click handler firing on the same click (e.g. a
GPX waypoint's normal popup) since this is a power-user/debugging
affordance, not a replacement for those. Feature properties can come from
an uploaded GPX/FIT file (untrusted input) via this app's own layers, so
every value is escaped the same way gpx.js's popups already are.

public/css/style.css

@@ -438,6 +438,40 @@ html, body {
   margin: 4px 0 0;
 }
 
+.debug-popup .maplibregl-popup-content {
+  max-height: 320px;
+  overflow-y: auto;
+}
+
+.debug-popup__feature {
+  font-size: 11px;
+  font-family: monospace;
+}
+
+.debug-popup__feature + .debug-popup__feature {
+  margin-top: 8px;
+  padding-top: 8px;
+  border-top: 1px solid #eee;
+}
+
+.debug-popup__feature table {
+  border-collapse: collapse;
+  width: 100%;
+}
+
+.debug-popup__feature th {
+  text-align: left;
+  color: #888;
+  font-weight: normal;
+  padding: 1px 6px 1px 0;
+  vertical-align: top;
+  white-space: nowrap;
+}
+
+.debug-popup__feature td {
+  word-break: break-word;
+}
+
 .gpx-popup__time {
   color: #888;
   font-size: 12px;