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/composables/useSchemaOptions.ts

@@ -14,7 +14,13 @@ export function useSchemaOptions() {
 
   function isSchemaAware(connectionId: string): boolean {
     const dbType = connectionStore.getConfig(connectionId)?.db_type;
-    return dbType === "postgres" || dbType === "sqlserver" || dbType === "oracle" || dbType === "dameng";
+    return (
+      dbType === "postgres" ||
+      dbType === "sqlserver" ||
+      dbType === "oracle" ||
+      dbType === "dameng" ||
+      dbType === "gaussdb"
+    );
   }
 
   async function loadSchemaOptions(connectionId: string, database: string) {