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/apple-icon.tsx

@@ -1,4 +1,5 @@
 import { ImageResponse } from "next/og";
+import { COLORS, BRAND } from "@/lib/constants";
 
 export const size = { width: 180, height: 180 };
 export const contentType = "image/png";
@@ -13,14 +14,14 @@ export default function AppleIcon() {
           display: "flex",
           alignItems: "center",
           justifyContent: "center",
-          background: "linear-gradient(135deg, #0a0e1a 0%, #111827 100%)",
+          background: `linear-gradient(135deg, ${COLORS.bgPrimary} 0%, ${COLORS.bgSecondary} 100%)`,
           borderRadius: "40px",
           fontSize: 120,
         }}
       >
-        🦞
+        {BRAND.emoji}
       </div>
     ),
-    { ...size }
+    size
   );
 }