feat: implement configuration management system

- Add comprehensive TypeScript interfaces for all config types
- Implement ConfigManager singleton class with layered loading
- Support for default, environment-specific, and environment variable configs
- Add example .env file for sensitive configuration

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

config/.env.example

@@ -0,0 +1,18 @@
+# SQL Server
+SQL_SERVER_SERVER=192.168.110.114
+SQL_SERVER_DATABASE=CompanyDB
+SQL_SERVER_USERNAME=peng
+SQL_SERVER_PASSWORD=your_password_here
+SQL_SERVER_DRIVER=ODBC Driver 18 for SQL Server
+SQL_SERVER_TRUST_CERT=yes
+
+# MySQL
+MYSQL_HOST=localhost
+MYSQL_PORT=3306
+MYSQL_DATABASE=erp_db
+MYSQL_USERNAME=root
+MYSQL_PASSWORD=your_password_here
+
+# ERP Credentials (optional, can also be entered in UI)
+ERP_USERNAME=BLDpengqiangqiang
+ERP_PASSWORD=your_password_here