ui: 暗色金融风重构 + 自定义货币选择器

- 深色主题：深蓝背景 + 毛玻璃卡片 + 金色结果数字
- 自定义底部弹窗货币选择器，替换原生白色 Picker
- 换算卡片内容居中竖排布局
- 汇率信息条改为竖排独立卡片
- 实时脉冲徽章 + 蓝紫渐变互换按钮

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

miniprogram/app.wxss

@@ -1,19 +1,23 @@
 page {
-  background: linear-gradient(180deg, #EEF2FF 0%, #F5F5F5 320rpx);
-  color: #1E293B;
-  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+  background: #0B1120;
+  color: #E2E8F0;
+  font-family: -apple-system, BlinkMacSystemFont, 'Helvetica Neue', sans-serif;
   font-size: 28rpx;
   line-height: 1.6;
 }
 
 .container {
-  padding: 32rpx;
-  padding-top: 24rpx;
+  min-height: 100vh;
+  padding: 24rpx 28rpx;
+  background:
+    radial-gradient(ellipse at 20% 0%, rgba(59, 130, 246, 0.08) 0%, transparent 50%),
+    radial-gradient(ellipse at 80% 100%, rgba(168, 85, 247, 0.06) 0%, transparent 50%),
+    #0B1120;
 }
 
 .card {
-  background-color: #FFFFFF;
-  border-radius: 24rpx;
+  background: rgba(30, 41, 59, 0.7);
+  border: 2rpx solid rgba(148, 163, 184, 0.1);
+  border-radius: 28rpx;
   margin-bottom: 24rpx;
-  box-shadow: 0 4rpx 24rpx rgba(0, 0, 0, 0.06);
 }