fix: P0 工程修复（eslint 9 / .env.example / 联系表单后端）

- eslint 迁移至 v9 flat config + eslint-config-next 16，lint 脚本改为 eslint .（next lint 在 Next 16 已移除），新增 typecheck 脚本
- React Compiler 严格规则对存量代码降为警告，修复邮件模板未转义字符
- .env.example 补齐 RESEND_VERIFIED_DOMAIN / RESEND_FROM_NAME / NEXT_PUBLIC_APP_NAME / CONTACT_RECEIVER_EMAIL
- email.ts 发件地址改为发送时求值，避免冷启动缓存错误值
- 联系表单接入 server action：留言落库 contact_message，并邮件通知管理员（HTML 转义防注入）

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -31,14 +31,15 @@
     "dev": "node scripts/run-dev.mjs",
     "build": "npm run generate:blog-manifest && next build",
     "start": "next start",
-    "lint": "next lint",
+    "lint": "eslint .",
     "db:generate": "drizzle-kit generate",
     "db:migrate": "drizzle-kit migrate",
     "db:push": "drizzle-kit push",
     "db:studio": "drizzle-kit studio",
     "admin:setup": "tsx scripts/setup-admin.ts",
     "seed:tools": "tsx scripts/seed-tools.ts",
-    "generate:blog-manifest": "node scripts/generate-blog-manifest.mjs"
+    "generate:blog-manifest": "node scripts/generate-blog-manifest.mjs",
+    "typecheck": "tsc --noEmit"
   },
   "dependencies": {
     "@aws-sdk/client-s3": "^3.958.0",
@@ -96,8 +97,8 @@
     "@types/react": "^19.2.7",
     "@types/react-dom": "^19.2.3",
     "dotenv": "^17.2.2",
-    "eslint": "^8",
-    "eslint-config-next": "14.2.3",
+    "eslint": "^9.39.5",
+    "eslint-config-next": "16.1.1",
     "pg": "^8.16.3",
     "playwright": "^1.57.0",
     "png-to-ico": "^3.0.1",