Gate firmware update tab behind ?beta=true (#127)

* Gate firmware update tab behind ?beta=true

* Use $ helper consistently in BETA_UI gate

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

docs/index.html

@@ -397,6 +397,12 @@ const $ = s=>document.querySelector(s);
 // -- only the UI is hidden, so adding ?debug=true after an unattended incident still shows the history.
 const DEBUG_UI = new URLSearchParams(location.search).get("debug")==="true";
 if(!DEBUG_UI) for(const el of document.querySelectorAll(".debugonly")) el.style.display="none";
+// Firmware-update tab gate: only visible with ?beta=true in the URL.
+const BETA_UI = new URLSearchParams(location.search).get("beta")==="true";
+if(!BETA_UI){
+  $('#mainTabs .slot-tab[data-tab="tabUpdate"]').style.display="none";
+  $("#updAvail").style.display="none";
+}
 function log(m){ const l=$("#log"); l.textContent=(new Date().toLocaleTimeString()+"  "+m+"\n"+l.textContent).slice(0,2000); }
 
 // ---- Loop-state trail (persistent) ----