[fix](trx-frontend-http): fix bookmark markers invisible on spectrum

let-declared bmList is not a window property, so window.bmList in
app.js always returned undefined. Change to var so it lands on window;
read it via typeof guard in app.js to stay safe if bookmarks.js is absent.

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/plugins/bookmarks.js

@@ -1,6 +1,6 @@
 // --- Bookmarks Tab ---
 
-let bmList = [];
+var bmList = [];
 let bmEditId = null;
 
 function bmFmtFreq(hz) {