feat: 添加龙虾 favicon 并修复 middleware 路由拦截

- 创建 lib/constants/theme.ts 统一管理颜色和品牌常量
- 重构 icon.tsx 和 apple-icon.tsx 使用共享常量
- 修复 middleware matcher 排除 icon/apple-icon 路由

app/icon.tsx

@@ -1,4 +1,5 @@
 import { ImageResponse } from "next/og";
+import { COLORS, BRAND } from "@/lib/constants";
 
 export const size = { width: 64, height: 64 };
 export const contentType = "image/png";
@@ -13,14 +14,14 @@ export default function Icon() {
           display: "flex",
           alignItems: "center",
           justifyContent: "center",
-          background: "#0a0e1a",
+          background: COLORS.bgPrimary,
           borderRadius: "14px",
           fontSize: 44,
         }}
       >
-        🦞
+        {BRAND.emoji}
       </div>
     ),
-    { ...size }
+    size
   );
 }