feat(panel): light/dark theme

panel/index.html

@@ -1,9 +1,20 @@
 <!doctype html>
-<html lang="en" class="dark">
+<html lang="en">
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Felis · Control Panel</title>
+    <script>
+      try {
+        const theme = localStorage.getItem("felis-theme") || "system";
+        const dark = theme === "dark" || (theme === "system" && window.matchMedia("(prefers-color-scheme: dark)").matches);
+        if (dark) {
+          document.documentElement.classList.add("dark");
+        } else {
+          document.documentElement.classList.remove("dark");
+        }
+      } catch (_) {}
+    </script>
   </head>
   <body>
     <div id="root"></div>