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/libs/bootstrap-select/sass/variables.scss

@@ -0,0 +1,9 @@
+$color-red-error: rgb(185, 74, 72);
+$color-grey-arrow: rgba(204, 204, 204, 0.2);
+
+$width-default: 220px; // 3 960px-grid columns
+
+$zindex-select-dropdown: 1060; // must be higher than a modal background (1050)
+
+//** Placeholder text color
+$input-color-placeholder: #999;