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.json

@@ -4,10 +4,10 @@
     "pages/overview/overview"
   ],
   "window": {
-    "navigationBarTitleText": "汇率换算",
-    "navigationBarBackgroundColor": "#FFFFFF",
-    "navigationBarTextStyle": "black",
-    "backgroundColor": "#F5F5F5",
+    "navigationBarTitleText": "",
+    "navigationBarBackgroundColor": "#0B1120",
+    "navigationBarTextStyle": "white",
+    "backgroundColor": "#0B1120",
     "backgroundTextStyle": "dark"
   },
   "style": "v2",