fix: 查询页两个细节修复

1. 点击侧边栏表名插入 db.table 格式(如 log_analytics.access_logs)
   - SchemaSidebar 表名: onInsert(t.name) → onInsert(`${dbName}.${t.name}`)
   - SchemaSidebar 字段名: onInsert(col.Field) → onInsert(`${t.name}.${col.Field}`)
2. 查询不存在的表应显示错误而非'无结果数据'
   - QueriesPage handleExecute: 检查 res.error 字段,有错误则 setError + 不设 result

frontend/src/pages/QueriesPage.tsx

@@ -210,7 +210,12 @@ export default function QueriesPage() {
     setExecuting(true); setError(''); setResult(null);
     try {
       const res = await queryApi.execute({ instance_id: Number(instanceId), database: database || undefined, sql }) as unknown as QueryResult;
-      setResult(res);
+      if (res.error) {
+        setError(res.error);
+        setResult(null);
+      } else {
+        setResult(res);
+      }
       setHistory(prev => [{ id: Date.now(), sql, instance_id: Number(instanceId), database, db_type: dbType, status: 'success', execution_time_ms: res.execution_time_ms || 0, result_preview: JSON.stringify(res.rows?.[0] || {}).substring(0, 100), created_at: new Date().toISOString(), favorite: false }, ...prev]);
     } catch (e: unknown) {
       const raw = (e as { response?: { data?: { detail?: string | Array<{ msg: string }> } } })?.response?.data?.detail;