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.js

@@ -1,7 +1,84 @@
+const { CURRENCIES, getPickerData } = require('../../utils/currencies');
+const { getRates, convert, formatAmount, formatRate } = require('../../utils/rate');
+
+const pickerData = getPickerData();
+
 Page({
-  data: {},
+  data: {
+    pickerData,
+    baseIndex: 0, // CNY
+    amount: '100',
+    list: [],
+    updatedAt: '',
+    loading: true,
+    error: '',
+    rates: null,
+  },
 
   onLoad() {
-    console.log('Overview page loaded');
+    this.loadRates();
+  },
+
+  onPullDownRefresh() {
+    this.loadRates().then(() => {
+      wx.stopPullDownRefresh();
+    });
+  },
+
+  async loadRates() {
+    this.setData({ loading: true, error: '' });
+    try {
+      const baseCode = CURRENCIES[this.data.baseIndex].code;
+      const { rates, updatedAt } = await getRates(baseCode);
+      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',
+      });
+    }
+  },
+
+  buildList() {
+    const { rates, baseIndex, amount } = this.data;
+    if (!rates) return;
+
+    const num = parseFloat(amount);
+    if (isNaN(num) || num <= 0) {
+      this.setData({ list: [] });
+      return;
+    }
+
+    const baseCode = CURRENCIES[baseIndex].code;
+    const list = CURRENCIES
+      .filter((c) => c.code !== baseCode)
+      .map((c) => {
+        const converted = convert(num, baseCode, c.code, rates);
+        const unitRate = convert(1, baseCode, c.code, rates);
+        return {
+          flag: c.flag,
+          code: c.code,
+          name: c.name,
+          converted: formatAmount(converted, c.code),
+          unitRate: formatRate(unitRate),
+        };
+      });
+
+    this.setData({ list });
+  },
+
+  handleBaseChange(e) {
+    this.setData({ baseIndex: Number(e.detail.value) });
+    this.loadRates();
+  },
+
+  handleAmountInput(e) {
+    this.setData({ amount: e.detail.value });
+    this.buildList();
   },
 });