feat: 添加 SVG 格式 favicon，匹配项目赛博朋克风格

frontend/index.html

@@ -3,6 +3,7 @@
 <head>
   <meta charset="UTF-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+  <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
   <title>CloudSSH</title>
   <script src="https://cdn.tailwindcss.com?plugins=forms,container-queries"></script>
   <link href="https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@400;600;700&display=swap" rel="stylesheet">

frontend/public/favicon.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <rect width="32" height="32" rx="4" fill="#131313"/>
+  <rect x="2" y="2" width="28" height="28" rx="3" fill="#1a1a1a" stroke="#4af626" stroke-width="1"/>
+  <path d="M8 11L14 16L8 21" stroke="#4af626" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+  <line x1="16" y1="21" x2="24" y2="21" stroke="#4af626" stroke-width="2.5" stroke-linecap="round"/>
+</svg>