[feat](trx-frontend-http): add tab navigation with Main and About tabs

Add a tab bar to the HTTP frontend card. All existing controls stay in
the Main tab. A new About tab shows server version, callsign, rig info,
client version, and connected client count, updated live via SSE.

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

@@ -289,6 +289,20 @@ function render(update) {
   }
 
   if (typeof update.clients === "number") lastClientCount = update.clients;
+  // Populate About tab
+  if (update.server_version) {
+    document.getElementById("about-server-ver").textContent = `trx-server v${update.server_version}`;
+  }
+  if (update.server_callsign) {
+    document.getElementById("about-server-call").textContent = update.server_callsign;
+  }
+  if (update.info) {
+    const parts = [update.info.manufacturer, update.info.model, update.info.revision].filter(Boolean).join(" ");
+    if (parts) document.getElementById("about-rig-info").textContent = parts;
+  }
+  if (typeof update.clients === "number") {
+    document.getElementById("about-clients").textContent = update.clients;
+  }
   powerHint.textContent = readyText();
   lastLocked = update.status && update.status.lock === true;
   lockBtn.textContent = lastLocked ? "Unlock" : "Lock";
@@ -568,6 +582,16 @@ lockBtn.addEventListener("click", async () => {
   }
 });
 
+// --- Tab navigation ---
+document.querySelector(".tab-bar").addEventListener("click", (e) => {
+  const btn = e.target.closest(".tab[data-tab]");
+  if (!btn) return;
+  document.querySelectorAll(".tab-bar .tab").forEach((t) => t.classList.remove("active"));
+  btn.classList.add("active");
+  document.querySelectorAll(".tab-panel").forEach((p) => p.style.display = "none");
+  document.getElementById(`tab-${btn.dataset.tab}`).style.display = "";
+});
+
 connect();
 
 // --- Signal measurement ---