feat: 汇率换算功能完整实现

- 首页双向换算：30种主流货币，实时计算，一键互换
- 汇率总览页：一对多查看所有货币换算结果
- 数据源 open.er-api.com，4小时本地缓存 + 失败兜底
- UI 美化：渐变背景、分色货币块、紫色互换按钮

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

miniprogram/pages/overview/overview.wxml

@@ -1,8 +1,64 @@
 <view class="container">
-  <view class="page-title">汇率总览</view>
-  <view class="page-subtitle">一个基准货币对所有主流货币</view>
+  <!-- 顶部控制区 -->
+  <view class="card header-card">
+    <view class="header-top">
+      <text class="header-title">📊 汇率总览</text>
+    </view>
 
-  <view class="card">
-    <view class="placeholder-text">功能开发中</view>
+    <view class="control-row">
+      <text class="control-label">基准</text>
+      <picker range="{{pickerData}}" value="{{baseIndex}}" bindchange="handleBaseChange">
+        <view class="base-tag">
+          <text class="base-tag-text">{{pickerData[baseIndex]}}</text>
+          <text class="tag-arrow">▼</text>
+        </view>
+      </picker>
+    </view>
+
+    <view class="control-row">
+      <text class="control-label">金额</text>
+      <input
+        class="amount-input"
+        type="digit"
+        value="{{amount}}"
+        bindinput="handleAmountInput"
+        placeholder="输入金额"
+        placeholder-class="amount-placeholder"
+      />
+    </view>
+
+    <view class="updated-at" wx:if="{{updatedAt}}">{{updatedAt}} 更新</view>
+  </view>
+
+  <!-- 错误提示 -->
+  <view class="card error-card" wx:if="{{error}}">
+    <text class="error-icon">⚠</text>
+    <text class="error-text">{{error}}</text>
+  </view>
+
+  <!-- 加载中 -->
+  <view class="loading-card" wx:if="{{loading}}">
+    <text>正在获取汇率数据...</text>
+  </view>
+
+  <!-- 汇率列表 -->
+  <view class="card list-card" wx:if="{{list.length}}">
+    <view
+      class="rate-item"
+      wx:for="{{list}}"
+      wx:key="code"
+    >
+      <view class="rate-left">
+        <text class="rate-flag">{{item.flag}}</text>
+        <view class="rate-info">
+          <text class="rate-code">{{item.code}}</text>
+          <text class="rate-name">{{item.name}}</text>
+        </view>
+      </view>
+      <view class="rate-right">
+        <text class="rate-value">{{item.converted}}</text>
+        <text class="rate-unit">1 = {{item.unitRate}}</text>
+      </view>
+    </view>
   </view>
 </view>