WebUI: MD3 2026 redesign + light/dark theme toggle

- Rewrite CSS with full MD3 color tokens + elevation + shape system
- Add light theme with data-theme="light"
- Theme toggle button in top bar (persists to localStorage)
- 268 lines CSS, covers login/shell/dashboard/plugins/projects/proxy

static/web_panel/home.html

@@ -12,6 +12,7 @@
             <div class="title">🐱 SenSu Alpha <span id="ver-badge" style="font-size:0.7em; opacity:0.5; margin-left:4px;"></span></div>
             <div class="user-info">
                 <span id="uname">Loading...</span>
+            <button class="theme-toggle" onclick="toggleTheme()" title="切换日夜间模式">🌓</button>
                 <button class="logout-btn" onclick="doLogout()">退出</button>
             </div>
         </header>
@@ -49,3 +50,18 @@
     <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>