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/skins/_all-skins.less

@@ -0,0 +1,19 @@
+//All skins in one file
+@import "skin-blue.less";
+@import "skin-blue-light.less";
+@import "skin-black.less";
+@import "skin-black-light.less";
+@import "skin-green.less";
+@import "skin-green-light.less";
+@import "skin-red.less";
+@import "skin-red-light.less";
+@import "skin-yellow.less";
+@import "skin-yellow-light.less";
+@import "skin-purple.less";
+@import "skin-purple-light.less";
+@import "skin-black-blue.less";
+@import "skin-black-purple.less";
+@import "skin-black-green.less";
+@import "skin-black-red.less";
+@import "skin-black-yellow.less";
+@import "skin-black-pink.less";