fix(shell): stale tabs now know the gateway was updated

Recurring confusion all day: the SPA lives in an open tab until reload, so
after a gateway rebuild/update users kept seeing (and benchmarking with!) an
old interface — "where is the feature", "why are the test ids old". The meta
endpoint now carries the process start timestamp; the existing 20s health
poll compares it, and when the gateway has restarted the tab shows a fixed
banner: "The gateway was updated — this page is running an older interface"
with a one-click Reload. 2 strings in all 7 languages.

web/js/app.js

@@ -256,12 +256,30 @@ function showLogin() {
   mount(document.getElementById("app"), renderLogin(boot));
 }
 
+let staleNotified = false;
+async function checkUiFresh() {
+  // the SPA lives in the tab until reload; if the gateway restarted (dev
+  // rebuild, update, config restart) the embedded UI may have changed —
+  // tell the user instead of letting them stare at stale views
+  try {
+    const m = await api.meta();
+    if (appState.meta?.started && m.started && m.started !== appState.meta.started && !staleNotified) {
+      staleNotified = true;
+      const bar = h("div", { class: "callout warn", style: "position:fixed; left:50%; transform:translateX(-50%); bottom:18px; z-index:99; box-shadow:0 8px 24px rgba(0,0,0,.35)" },
+        h("span", {}, "🔄 " + t("shell.uiStale")), " ",
+        h("button", { class: "btn sm primary", onClick: () => location.reload() }, t("shell.reload")));
+      document.body.append(bar);
+    }
+  } catch {}
+}
+
 async function refreshHealth() {
   try {
     appState.health = await api.health();
   } catch {
     appState.health = null;
   }
+  checkUiFresh();
   // re-render topbar only if shell present
   const route = currentRoute();
   const main = document.querySelector(".main");