Fix theme toggle: move init to app.js, remove duplicate script

- Theme initialized BEFORE page render (no flash)
- toggleTheme() now in app.js global scope
- Proxy page rewritten with btn-filled MD3 class

static/web_panel/home.html

@@ -58,18 +58,3 @@
     <script src="./static/js/app.js"></script>
 </body>
 </html>
-
-<script>
-function toggleTheme(){
-  var t=document.documentElement.getAttribute("data-theme")==="light"?"dark":"light";
-  document.documentElement.setAttribute("data-theme",t);
-  localStorage.setItem("sensu-theme",t);
-  document.querySelector(".theme-toggle").textContent=t==="light"?"☀️":"🌙";
-}
-(function(){
-  var s=localStorage.getItem("sensu-theme")||"dark";
-  document.documentElement.setAttribute("data-theme",s);
-  var btn=document.querySelector(".theme-toggle");
-  if(btn) btn.textContent=s==="light"?"☀️":"🌙";
-})();
-</script>