Web: poll for device reboot after OTA upload

After a successful firmware upload the UI previously showed a static
"Done. Device is rebooting..." message with no indication of progress.

Replace the static message with a live polling loop.  A dot is appended
to the message every second as a visual progress indicator.  After a
4-second initial delay (to allow the device to start rebooting), the
loop fetches "/" to check whether the HTTPS server is back up.  On a
successful response the interval is cleared and the user is redirected
to the login page after 1.5 s (the session token is invalidated by the
reboot).  On a network error the fetch is retried every 4 seconds.
A guard value of 1000 prevents a second fetch from being started while
the first is still in flight.  If the device does not respond within
60 seconds the loop stops and an error message is shown.

src/helpers/web/WebPanelServer.cpp

@@ -2641,7 +2641,23 @@ const char kWebPanelAppHtml[] PROGMEM = R"HTML(
         }
         if (xhr.status === 200) {
           barEl.style.width = "100%";
-          msgEl.textContent = "Done. Device is rebooting...";
+          msgEl.textContent = "Done. Device is rebooting";
+          let wait = 4;
+          const iv = setInterval(async () => {
+            msgEl.textContent += '.';
+            if (msgEl.textContent.length > 85) { clearInterval(iv); msgEl.textContent = "Device did not come back online. =("; return; }
+            if (--wait) { return; }
+            wait = 1000; // prevent concurrent fetch
+            try {
+              if ((await fetch("/", { cache: "no-store" })).ok) {
+                clearInterval(iv);
+                msgEl.textContent = "Device is back online. Redirecting to login...";
+                setTimeout(redirectToLogin, 1500);
+                return;
+              }
+            } catch (_) {}
+            wait = 4;
+          }, 1000);
         } else {
           msgEl.textContent = "Failed: " + (xhr.responseText || String(xhr.status));
           document.getElementById("otaUploadBtn").disabled = false;