[fix](trx-frontend-http): hide tabs until authentication is granted

Hide the Main/Plugins/About tab bar initially, only showing it after
the user successfully authenticates. This prevents navigation options
from being visible when access has not been granted.

Changes:
- Add display:none and id to tab-bar div in index.html
- Update showAuthGate() to hide tab-bar
- Update hideAuthGate() to show tab-bar

Now the UI flow is:
1. Header only (auth gate visible)
2. After login: Header + tabs + content

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/app.js

@@ -58,11 +58,13 @@ function showAuthGate() {
   document.getElementById("loading").style.display = "none";
   document.getElementById("content").style.display = "none";
   document.getElementById("auth-gate").style.display = "block";
+  document.getElementById("tab-bar").style.display = "none";
 }
 
 function hideAuthGate() {
   document.getElementById("auth-gate").style.display = "none";
   document.getElementById("loading").style.display = "block";
+  document.getElementById("tab-bar").style.display = "";
 }
 
 function showAuthError(msg) {