[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/app.js

@@ -86,14 +86,23 @@ function updateAuthUI() {
   const btn = document.getElementById("logout-btn");
   const badge = document.getElementById("auth-badge");
   const badgeRole = document.getElementById("auth-role-badge");
+  const headerAuthBtn = document.getElementById("header-auth-btn");
 
   if (authRole) {
     btn.style.display = "block";
     badge.style.display = "block";
     badgeRole.textContent = authRole === "control" ? "Control (full access)" : "RX (read-only)";
+    if (headerAuthBtn) {
+      headerAuthBtn.textContent = "Logout";
+      headerAuthBtn.style.display = "block";
+    }
   } else {
     btn.style.display = "none";
     badge.style.display = "none";
+    if (headerAuthBtn) {
+      headerAuthBtn.textContent = "Login";
+      headerAuthBtn.style.display = "block";
+    }
   }
 }
 
@@ -1220,6 +1229,22 @@ document.getElementById("logout-btn").addEventListener("click", async () => {
   }
 });
 
+// Setup header auth button (Login/Logout)
+const headerAuthBtn = document.getElementById("header-auth-btn");
+if (headerAuthBtn) {
+  headerAuthBtn.addEventListener("click", async () => {
+    if (authRole) {
+      // Logged in - show logout confirmation
+      if (confirm("Are you sure you want to logout?")) {
+        await authLogout();
+      }
+    } else {
+      // Not logged in - show auth gate
+      showAuthGate(false);
+    }
+  });
+}
+
 // Start the app
 initializeApp();
 window.addEventListener("resize", resizeHeaderSignalCanvas);