feat: add MySQL database support alongside SQL Server

This commit implements multi-database support, allowing the system to switch
between SQL Server and MySQL databases seamlessly.

## New Features
- Database type selection (SQL Server or MySQL) via configuration
- Automatic table name conversion between formats ([dbo].[table] → dbo_table)
- Automatic parameter placeholder handling (? for SQL Server, %s for MySQL)
- GUI settings tab now includes database type dropdown and MySQL configuration

## Database Abstraction Layer
- db/base_connection.py: Abstract base class for database connections
- db/sqlserver_connection.py: SQL Server implementation
- db/mysql_connection.py: MySQL implementation using mysql-connector-python
- db/connection_factory.py: Factory pattern for creating connections
- db/table_name_converter.py: Table name format conversion utility

## DAO Base Class
- db/base_dao.py: Base DAO with helper methods for SQL conversion and placeholders

## Updated Components
- config/schema.py: Extended with DatabaseType enum and MySQL/SQLServer config classes
- config/defaults.py: Added MySQL default configuration
- config/loader.py: Updated to handle new database structure
- db/connection.py: Refactored to use factory pattern and load user config
- All DAO files: Updated to inherit from BaseDAO with automatic conversion

## Dependencies
- Added mysql-connector-python>=8.0.0 to requirements.txt

## Configuration
To use MySQL, set db_type to "mysql" in config/user_settings.json:
{
  "database": {
    "db_type": "mysql",
    "mysql": {
      "host": "192.168.31.83",
      "port": 3306,
      "database": "BLD_DB",
      "username": "remote_user",
      "password": "3.1415926Beeke"
    }
  }
}

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

config/schema.py

@@ -8,6 +8,13 @@
 from dataclasses import dataclass, field
 from typing import Optional
 from pathlib import Path
+from enum import Enum
+
+
+class DatabaseType(str, Enum):
+    """数据库类型枚举"""
+    SQLSERVER = "sqlserver"
+    MYSQL = "mysql"
 
 
 @dataclass
@@ -33,28 +40,56 @@ class ERPConfig:
         return errors
 
 
+@dataclass
+class SQLServerConfig:
+    """SQL Server 特定配置"""
+    driver: str = "ODBC Driver 18 for SQL Server"
+    trust_server_certificate: str = "yes"
+
+
+@dataclass
+class MySQLConfig:
+    """MySQL 特定配置"""
+    host: str = ""
+    port: int = 3306
+    charset: str = "utf8mb4"
+
+
 @dataclass
 class DatabaseConfig:
     """数据库配置"""
 
-    server: str
-    database: str
-    username: str
-    password: str
-    driver: str = "ODBC Driver 18 for SQL Server"
-    trust_server_certificate: str = "yes"
+    db_type: DatabaseType = DatabaseType.SQLSERVER
+    server: str = ""  # SQL Server 服务器地址
+    database: str = ""
+    username: str = ""
+    password: str = ""
+    sqlserver: Optional[SQLServerConfig] = None
+    mysql: Optional[MySQLConfig] = None
 
     def validate(self) -> list[str]:
         """验证配置，返回错误列表"""
         errors = []
-        if not self.server:
-            errors.append("数据库服务器地址不能为空")
-        if not self.database:
-            errors.append("数据库名称不能为空")
-        if not self.username:
-            errors.append("数据库用户名不能为空")
-        if not self.password:
-            errors.append("数据库密码不能为空")
+
+        if self.db_type == DatabaseType.SQLSERVER:
+            if not self.server:
+                errors.append("SQL Server 服务器地址不能为空")
+            if not self.database:
+                errors.append("数据库名称不能为空")
+            if not self.username:
+                errors.append("数据库用户名不能为空")
+            if not self.password:
+                errors.append("数据库密码不能为空")
+        elif self.db_type == DatabaseType.MYSQL:
+            if self.mysql and not self.mysql.host:
+                errors.append("MySQL 主机地址不能为空")
+            if not self.database:
+                errors.append("数据库名称不能为空")
+            if not self.username:
+                errors.append("数据库用户名不能为空")
+            if not self.password:
+                errors.append("数据库密码不能为空")
+
         return errors
 
 
@@ -171,12 +206,20 @@ class AppConfig:
                 "auto_close_browser": self.erp.auto_close_browser,
             },
             "database": {
+                "db_type": self.database.db_type.value,
                 "server": self.database.server,
                 "database": self.database.database,
                 "username": self.database.username,
                 "password": self.database.password,
-                "driver": self.database.driver,
-                "trust_server_certificate": self.database.trust_server_certificate,
+                "sqlserver": {
+                    "driver": self.database.sqlserver.driver if self.database.sqlserver else "ODBC Driver 18 for SQL Server",
+                    "trust_server_certificate": self.database.sqlserver.trust_server_certificate if self.database.sqlserver else "yes",
+                },
+                "mysql": {
+                    "host": self.database.mysql.host if self.database.mysql else "",
+                    "port": self.database.mysql.port if self.database.mysql else 3306,
+                    "charset": self.database.mysql.charset if self.database.mysql else "utf8mb4",
+                },
             },
             "paths": {
                 "data_dir": self.paths.data_dir,