[feat](trx-frontend-http): add Plugins tab showing registered frontends

Add GET /frontends API endpoint returning registered frontend names as
JSON. Add Plugins tab to the web UI that fetches and displays the list.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -19,6 +19,7 @@
     </div>
     <div class="tab-bar">
       <button class="tab active" data-tab="main">Main</button>
+      <button class="tab" data-tab="plugins">Plugins</button>
       <button class="tab" data-tab="about">About</button>
     </div>
     <div id="tab-main" class="tab-panel">
@@ -116,6 +117,9 @@
     </div>
     </div>
     </div>
+    <div id="tab-plugins" class="tab-panel" style="display:none;">
+      <div id="plugins-list"></div>
+    </div>
     <div id="tab-about" class="tab-panel" style="display:none;">
       <table class="about-table">
         <tr><td>Server</td><td id="about-server-ver">--</td></tr>