[style](trx-frontend): theme receiver marker via CSS var

Remove hardcoded #3388ff from the TRX circleMarker; apply
.trx-receiver-marker class and use stroke/fill: var(--accent-green)
so the dot follows the active colour scheme.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js

@@ -2447,7 +2447,7 @@ function initAprsMap() {
 
   if (hasLocation) {
     aprsMapReceiverMarker = L.circleMarker([serverLat, serverLon], {
-      radius: 8, color: "#3388ff", fillColor: "#3388ff", fillOpacity: 0.8
+      radius: 8, className: "trx-receiver-marker", fillOpacity: 0.8
     }).addTo(aprsMap).bindPopup("");
   }