feat(query): set schema context (search_path) before query execution (#113)

Add schema selector to the editor toolbar for schema-aware databases
(PostgreSQL, SQL Server, Oracle). When a schema is selected, the backend
sets the appropriate search_path before executing queries so users no
longer need to write schema-qualified table names.

src/types/database.ts

@@ -147,6 +147,7 @@ export interface QueryTab {
   title: string;
   connectionId: string;
   database: string;
+  schema?: string;
   sql: string;
   lastExecutedSql?: string;
   pinned?: boolean;