feat: 后端全量更新 - 含所有本次需求

- Contract.php: 返回合约账户余额(balance_contract)
- My.php: 地址管理增加BTC/ETH
- AppContract.php: 一键平仓(closeall)
- AppProxy.php: 代理专属注册链接 + 分级权限(L1/L2)
- site.php: 手续费减半(0.018→0.009)
- agent_permission_setup.sql: 代理权限SQL
- crypto_news_crawler.py: 新闻自动采集脚本

public/assets/less/bootstrap-less/mixins/responsive-visibility.less

@@ -0,0 +1,21 @@
+// Responsive utilities
+
+//
+// More easily include all the states for responsive-utilities.less.
+.responsive-visibility() {
+  display: block !important;
+  table& {
+    display: table !important;
+  }
+  tr& {
+    display: table-row !important;
+  }
+  th&,
+  td& {
+    display: table-cell !important;
+  }
+}
+
+.responsive-invisibility() {
+  display: none !important;
+}