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/composables/useSchemaOptions.ts

@@ -0,0 +1,50 @@
+import { ref } from "vue";
+import { useConnectionStore } from "@/stores/connectionStore";
+import * as api from "@/lib/api";
+
+export function useSchemaOptions() {
+  const connectionStore = useConnectionStore();
+
+  const schemaOptions = ref<Record<string, string[]>>({});
+  const loadingSchemaOptions = ref<Record<string, boolean>>({});
+
+  function cacheKey(connectionId: string, database: string) {
+    return `${connectionId}:${database}`;
+  }
+
+  function isSchemaAware(connectionId: string): boolean {
+    const dbType = connectionStore.getConfig(connectionId)?.db_type;
+    return dbType === "postgres" || dbType === "sqlserver" || dbType === "oracle";
+  }
+
+  async function loadSchemaOptions(connectionId: string, database: string) {
+    if (!isSchemaAware(connectionId) || !database) return;
+    const key = cacheKey(connectionId, database);
+    if (loadingSchemaOptions.value[key]) return;
+
+    loadingSchemaOptions.value[key] = true;
+    try {
+      await connectionStore.ensureConnected(connectionId);
+      schemaOptions.value[key] = await api.listSchemas(connectionId, database);
+    } finally {
+      loadingSchemaOptions.value[key] = false;
+    }
+  }
+
+  function getSchemaOptionsForDb(connectionId: string, database: string): string[] {
+    return schemaOptions.value[cacheKey(connectionId, database)] ?? [];
+  }
+
+  function isLoadingSchemas(connectionId: string, database: string): boolean {
+    return !!loadingSchemaOptions.value[cacheKey(connectionId, database)];
+  }
+
+  return {
+    schemaOptions,
+    loadingSchemaOptions,
+    loadSchemaOptions,
+    getSchemaOptionsForDb,
+    isLoadingSchemas,
+    isSchemaAware,
+  };
+}