Add Access as a new database type with agent-based connectivity, UCanAccess JDBC URL parsing, backtick identifier quoting, file-path connection dialog, and PNG icons for Access/GoldenDB/Vastbase.
## 修改内容 - 新增 connectionUrlPlaceholder 纯函数于 connectionPresentation.ts,根据 db_type 返回对应 URL 格式 - ConnectionDialog.vue 中引用该函数替换原有统一的 i18n placeholder - 新增 20 个单元测试覆盖所有数据库类型及边界场景