[fix](trx-frontend-http): guard initAprsMap against missing Leaflet variable

Prevent ReferenceError when navigating to the map tab before the
Leaflet CDN script has finished loading.

https://claude.ai/code/session_018nDze1zN1AR3UgYRx5pqcL
Signed-off-by: Claude <noreply@anthropic.com>

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

@@ -6030,6 +6030,7 @@ function materializeBufferedMapLayers() {
 }
 
 function initAprsMap() {
+  if (typeof L === "undefined") return;
   const mapEl = document.getElementById("aprs-map");
   if (!mapEl) return;
   sizeAprsMapToViewport();