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/font-awesome/scss/_stacked.scss

@@ -0,0 +1,20 @@
+// Stacked Icons
+// -------------------------
+
+.#{$fa-css-prefix}-stack {
+  position: relative;
+  display: inline-block;
+  width: 2em;
+  height: 2em;
+  line-height: 2em;
+  vertical-align: middle;
+}
+.#{$fa-css-prefix}-stack-1x, .#{$fa-css-prefix}-stack-2x {
+  position: absolute;
+  left: 0;
+  width: 100%;
+  text-align: center;
+}
+.#{$fa-css-prefix}-stack-1x { line-height: inherit; }
+.#{$fa-css-prefix}-stack-2x { font-size: 2em; }
+.#{$fa-css-prefix}-inverse { color: $fa-inverse; }