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/index/index.js

@@ -1,7 +1,106 @@
+const { CURRENCIES, getPickerData } = require('../../utils/currencies');
+const { getRates, convert, formatAmount, formatRate } = require('../../utils/rate');
+
+const pickerData = getPickerData();
+const DEFAULT_FROM = 0;  // CNY
+const DEFAULT_TO = 1;    // USD
+
 Page({
-  data: {},
+  data: {
+    pickerData,
+    fromIndex: DEFAULT_FROM,
+    toIndex: DEFAULT_TO,
+    amount: '100',
+    result: '',
+    unitRate: '',
+    reverseRate: '',
+    updatedAt: '',
+    loading: true,
+    error: '',
+    rates: null,
+    baseCurrency: 'CNY',
+  },
 
   onLoad() {
-    console.log('Index page loaded');
+    this.loadRates();
+  },
+
+  onPullDownRefresh() {
+    this.loadRates().then(() => {
+      wx.stopPullDownRefresh();
+    });
+  },
+
+  async loadRates() {
+    this.setData({ loading: true, error: '' });
+    try {
+      const { rates, updatedAt } = await getRates('CNY');
+      this.setData({
+        rates,
+        baseCurrency: 'CNY',
+        updatedAt,
+        loading: false,
+      });
+      this.calculate();
+    } catch (e) {
+      this.setData({
+        loading: false,
+        error: '汇率获取失败，请检查网络后下拉刷新',
+      });
+    }
+  },
+
+  calculate() {
+    const { rates, fromIndex, toIndex, amount, baseCurrency } = this.data;
+    if (!rates) return;
+
+    const num = parseFloat(amount);
+    if (isNaN(num) || num <= 0) {
+      this.setData({ result: '', unitRate: '', reverseRate: '' });
+      return;
+    }
+
+    const fromCode = CURRENCIES[fromIndex].code;
+    const toCode = CURRENCIES[toIndex].code;
+    const converted = convert(num, fromCode, toCode, rates);
+    const unitForward = convert(1, fromCode, toCode, rates);
+    const unitReverse = convert(1, toCode, fromCode, rates);
+
+    this.setData({
+      result: formatAmount(converted, toCode),
+      unitRate: `1 ${fromCode} = ${formatRate(unitForward)} ${toCode}`,
+      reverseRate: `1 ${toCode} = ${formatRate(unitReverse)} ${fromCode}`,
+    });
+  },
+
+  handleAmountInput(e) {
+    const value = e.detail.value;
+    this.setData({ amount: value });
+    this.calculate();
+  },
+
+  handleFromChange(e) {
+    this.setData({ fromIndex: Number(e.detail.value) });
+    this.calculate();
+  },
+
+  handleToChange(e) {
+    this.setData({ toIndex: Number(e.detail.value) });
+    this.calculate();
+  },
+
+  handleSwap() {
+    const { fromIndex, toIndex } = this.data;
+    this.setData({
+      fromIndex: toIndex,
+      toIndex: fromIndex,
+    });
+    this.calculate();
+  },
+
+  goOverview() {
+    wx.navigateTo({
+      url: '/pages/overview/overview',
+    });
   },
 });