docs: 添加离线安装说明和 SVG 图标

browser-extension/icons/icon.svg

@@ -0,0 +1,27 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128">
+  <defs>
+    <linearGradient id="bg" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#667eea"/>
+      <stop offset="100%" style="stop-color:#764ba2"/>
+    </linearGradient>
+  </defs>
+  <rect width="128" height="128" rx="24" fill="url(#bg)"/>
+  <g fill="white">
+    <!-- 文档图标 -->
+    <rect x="36" y="24" width="56" height="72" rx="4" fill="white" opacity="0.9"/>
+    <!-- 文字线条 -->
+    <rect x="44" y="36" width="32" height="4" rx="2" fill="#667eea"/>
+    <rect x="44" y="48" width="40" height="4" rx="2" fill="#667eea" opacity="0.6"/>
+    <rect x="44" y="60" width="36" height="4" rx="2" fill="#667eea" opacity="0.6"/>
+    <rect x="44" y="72" width="28" height="4" rx="2" fill="#667eea" opacity="0.6"/>
+    <!-- 魔法棒 -->
+    <g transform="translate(70, 70) rotate(-45)">
+      <rect x="0" y="0" width="8" height="40" rx="2" fill="#FFD700"/>
+      <polygon points="4,-8 0,0 8,0" fill="#FFD700"/>
+    </g>
+    <!-- 星星 -->
+    <circle cx="100" cy="32" r="4" fill="#FFD700"/>
+    <circle cx="108" cy="48" r="3" fill="#FFD700" opacity="0.7"/>
+    <circle cx="96" cy="52" r="2" fill="#FFD700" opacity="0.5"/>
+  </g>
+</svg>