feat(connection): 支持设置默认数据库

统一新建查询的默认数据库选择逻辑，减少重复手动选择。

src/lib/defaultDatabase.ts

@@ -0,0 +1,12 @@
+import type { ConnectionConfig } from "@/types/database";
+
+export function resolveDefaultDatabase(connection: Pick<ConnectionConfig, "database">, options: string[]): string {
+  return connection.database || options[0] || "";
+}
+
+export function isDefaultDatabase(
+  connection: Pick<ConnectionConfig, "database"> | undefined,
+  database: string,
+): boolean {
+  return !!connection?.database && !!database && connection.database === database;
+}