🐛 fix(CreateDatabase): 修复 Oracle 新建数据库时因缺少 Service Name 报错

- 前端 Oracle/达梦连接保留原始 database 字段而非清空
- 后端添加 Oracle/达梦不支持此入口创建的友好提示
- refs #223

frontend/src/components/Sidebar.tsx

@@ -2189,7 +2189,7 @@ const Sidebar: React.FC<{ onEditConnection?: (conn: SavedConnection) => void }>
               ...conn.config, 
               port: Number(conn.config.port),
               password: conn.config.password || "",
-              database: "", // No db selected
+              database: (conn.config.type === 'oracle' || conn.config.type === 'dameng') ? (conn.config.database || "") : "",
               useSSH: conn.config.useSSH || false,
               ssh: conn.config.ssh || { host: "", port: 22, user: "", password: "", keyPath: "" }
           };