[feat](trx-frontend-http): add header auth button (Login/Logout)

Add a Login/Logout button in the header next to the theme toggle,
styled consistently with the theme button. Button behavior:

- When logged in: Shows "Logout" with confirmation
- When not logged in: Shows "Login" to open auth gate
- Visible when on main app (not in auth gate)
- Same theme-toggle-btn styling

Provides quick access to authentication controls without needing to
navigate to the About tab.

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

@@ -23,6 +23,7 @@
       <div class="header-right">
         <img id="logo" class="header-logo" src="/logo.png?v=1" alt="trx logo" onerror="this.style.display='none'" />
         <button id="theme-toggle" class="theme-toggle-btn" type="button" aria-label="Toggle dark or light theme">Light</button>
+        <button id="header-auth-btn" class="theme-toggle-btn" type="button" style="display:none;" aria-label="Login or Logout">Login</button>
       </div>
     </div>
     <!-- Auth gate (hidden by default, shown if auth is required) -->