feat: add GaussDB ODBC support and flatten connection list

- Add GaussDB/openGauss support via ODBC (resolves #106)
- Use pg_catalog queries instead of information_schema for compatibility
- Share ODBC environment across drivers with auto ODBCSYSINI detection
- Fix ODBC error message false positive on "Driver" keyword match
- Merge connection dialog categories into a single flat list
- Remove duplicate DDL button from data grid footer

src/components/diagram/SchemaDiagramDialog.vue

@@ -49,8 +49,17 @@ const props = defineProps<{
   focusTableName?: string;
 }>();
 
-const SQL_TYPES: DatabaseType[] = ["mysql", "postgres", "sqlite", "sqlserver", "oracle", "redshift", "dameng"];
-const SCHEMA_AWARE_TYPES: DatabaseType[] = ["postgres", "sqlserver", "oracle", "redshift", "dameng"];
+const SQL_TYPES: DatabaseType[] = [
+  "mysql",
+  "postgres",
+  "sqlite",
+  "sqlserver",
+  "oracle",
+  "redshift",
+  "dameng",
+  "gaussdb",
+];
+const SCHEMA_AWARE_TYPES: DatabaseType[] = ["postgres", "sqlserver", "oracle", "redshift", "dameng", "gaussdb"];
 const CARD_WIDTH = 270;
 const COLUMN_ROW_HEIGHT = 24;
 const CARD_HEADER_HEIGHT = 44;