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>

.env.example

@@ -36,12 +36,20 @@ RESEND_API_KEY="re_your_api_key"
 
 # Email Configuration - Choose one of the following options:
 #
-# Set a complete from email address (recommended, highest priority)
+# Option A (recommended, highest priority): set a complete from email address
 RESEND_FROM_EMAIL="Your App <noreply@yourdomain.com>"
 
+# Option B: set a verified domain (production requires A or B),
+# from address becomes "{RESEND_FROM_NAME} <noreply@{RESEND_VERIFIED_DOMAIN}>"
+RESEND_VERIFIED_DOMAIN=""
+RESEND_FROM_NAME=""
+
 # Optional: Resend Audience ID for newsletter/marketing
 RESEND_AUDIENCE_ID="your-resend-audience-id"
 
+# Optional: where contact form notifications are sent (admin inbox)
+CONTACT_RECEIVER_EMAIL=""
+
 # ====================================
 # Analytics & Monitoring (Optional)
 # ====================================
@@ -75,6 +83,9 @@ STORAGE_PUBLIC_URL=""
 # Public app URL (update in production)
 NEXT_PUBLIC_APP_URL="http://localhost:3000"
 
+# App name used in email sender and titles (optional)
+NEXT_PUBLIC_APP_NAME="Toolso.AI"
+
 # Node environment
 NODE_ENV="development"