[fix](trx-frontend-http): use dedicated --ais-accent color for map markers

Replace --accent-red with a new --ais-accent CSS variable (default #00aacc
cyan-blue) so AIS vessel markers and track lines are visually distinct from
other UI elements regardless of theme. Light theme uses a slightly darker
#0088aa for readability on the map.

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

@@ -5014,7 +5014,7 @@ function syncSelectedAisTrackVisibility() {
 }
 
 function getAisAccentColor() {
-  return getComputedStyle(document.documentElement).getPropertyValue("--accent-red").trim() || "#ff7559";
+  return getComputedStyle(document.documentElement).getPropertyValue("--ais-accent").trim() || "#00aacc";
 }
 
 function aisMarkerOptionsFromMessage(msg) {