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

lib/constants/theme.ts

@@ -0,0 +1,10 @@
+// Theme constants shared across app (including ImageResponse which cannot use CSS variables)
+
+export const COLORS = {
+  bgPrimary: "#0a0e1a",
+  bgSecondary: "#111827",
+} as const;
+
+export const BRAND = {
+  emoji: "🦞",
+} as const;