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/js/app.js

@@ -1,5 +1,22 @@
+// 主题初始化 (必须在渲染前执行, 防止闪白)
+(function(){
+  var s = localStorage.getItem("sensu-theme") || "dark";
+  document.documentElement.setAttribute("data-theme", s);
+})();
+
+window.toggleTheme = function(){
+  var t = document.documentElement.getAttribute("data-theme") === "light" ? "dark" : "light";
+  document.documentElement.setAttribute("data-theme", t);
+  localStorage.setItem("sensu-theme", t);
+  var btn = document.querySelector(".theme-toggle");
+  if(btn) btn.textContent = t === "light" ? "☀️" : "🌙";
+};
+
 // 初始化检查
 window.onload = async () => {
+  // 设置按钮初始图标
+  var btn = document.querySelector(".theme-toggle");
+  if(btn) btn.textContent = document.documentElement.getAttribute("data-theme") === "light" ? "☀️" : "🌙";
     try {
         const res = await fetch('./api/auth/status', { credentials: 'include' });
         if(res.status === 401) { window.location.href = './index.html'; return; }