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/pages/overview/overview.js

@@ -6,13 +6,14 @@ const pickerData = getPickerData();
 Page({
   data: {
     pickerData,
-    baseIndex: 0, // CNY
+    baseIndex: 0,
     amount: '100',
     list: [],
     updatedAt: '',
     loading: true,
     error: '',
     rates: null,
+    showPicker: false,
   },
 
   onLoad() {
@@ -30,17 +31,10 @@ Page({
     try {
       const baseCode = CURRENCIES[this.data.baseIndex].code;
       const { rates, updatedAt } = await getRates(baseCode);
-      this.setData({
-        rates,
-        updatedAt,
-        loading: false,
-      });
+      this.setData({ rates, updatedAt, loading: false });
       this.buildList();
     } catch (e) {
-      this.setData({
-        loading: false,
-        error: '汇率获取失败，请检查网络后下拉刷新',
-      });
+      this.setData({ loading: false, error: '汇率获取失败，请下拉刷新重试' });
     }
   },
 
@@ -72,11 +66,19 @@ Page({
     this.setData({ list });
   },
 
-  handleBaseChange(e) {
-    this.setData({ baseIndex: Number(e.detail.value) });
+  openBasePicker() {
+    this.setData({ showPicker: true });
+  },
+
+  handlePickerSelect(e) {
+    this.setData({ baseIndex: e.detail.value, showPicker: false });
     this.loadRates();
   },
 
+  handlePickerClose() {
+    this.setData({ showPicker: false });
+  },
+
   handleAmountInput(e) {
     this.setData({ amount: e.detail.value });
     this.buildList();