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/mixins.less

@@ -0,0 +1,40 @@
+// Mixins
+// --------------------------------------------------
+
+// Utilities
+@import "mixins/hide-text.less";
+@import "mixins/opacity.less";
+@import "mixins/image.less";
+@import "mixins/labels.less";
+@import "mixins/reset-filter.less";
+@import "mixins/resize.less";
+@import "mixins/responsive-visibility.less";
+@import "mixins/size.less";
+@import "mixins/tab-focus.less";
+@import "mixins/reset-text.less";
+@import "mixins/text-emphasis.less";
+@import "mixins/text-overflow.less";
+@import "mixins/vendor-prefixes.less";
+
+// Components
+@import "mixins/alerts.less";
+@import "mixins/buttons.less";
+@import "mixins/panels.less";
+@import "mixins/pagination.less";
+@import "mixins/list-group.less";
+@import "mixins/nav-divider.less";
+@import "mixins/forms.less";
+@import "mixins/progress-bar.less";
+@import "mixins/table-row.less";
+
+// Skins
+@import "mixins/background-variant.less";
+@import "mixins/border-radius.less";
+@import "mixins/gradients.less";
+
+// Layout
+@import "mixins/clearfix.less";
+@import "mixins/center-block.less";
+@import "mixins/nav-vertical-align.less";
+@import "mixins/grid-framework.less";
+@import "mixins/grid.less";