[refactor](trx-frontend-http): consolidate to single auth interface

Remove duplicate logout button from About tab. Use only the header
Login/Logout button for unified authentication control.

The About tab now shows the authentication badge (when logged in)
without the redundant logout button.

Single login view:
- Auth gate with Login form + Continue as Guest button (when no rx pass)
- Header Login/Logout button for quick access
- Auth badge in About tab showing current role

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

@@ -239,10 +239,7 @@
       </div>
     </div>
     <div id="tab-about" class="tab-panel" style="display:none;">
-      <div style="margin-bottom: 1rem;">
-        <button id="logout-btn" type="button" style="display:none; padding: 0.5rem 1rem; background: var(--accent-red, #ff6b6b); color: var(--bg-primary); border: none; border-radius: 0.25rem; font-weight: 600; cursor: pointer;">Logout</button>
-        <div id="auth-badge" style="display:none; margin-top: 0.5rem; color: var(--text-muted); font-size: 0.85rem;">Authenticated as: <strong id="auth-role-badge">--</strong></div>
-      </div>
+      <div id="auth-badge" style="display:none; margin-bottom: 1rem; padding: 0.5rem; background: var(--bg-secondary); border-radius: 0.25rem; color: var(--text-muted); font-size: 0.85rem;">Authenticated as: <strong id="auth-role-badge">--</strong></div>
       <table class="about-table">
         <tr><td>Server version</td><td id="about-server-ver">--</td></tr>
         <tr><td>Server address</td><td id="about-server-addr">--</td></tr>