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/utils/rate.js

@@ -0,0 +1,130 @@
+/**
+ * 汇率 API 调用 + 本地缓存
+ * 数据源：https://open.er-api.com/v6/latest/{base}
+ * 免费，无需 key，每日更新，1500 次/月
+ */
+
+const CACHE_KEY = 'exchange_rates';
+const CACHE_TTL = 4 * 60 * 60 * 1000; // 4 小时
+
+function getCache() {
+  try {
+    const data = wx.getStorageSync(CACHE_KEY);
+    if (!data) return null;
+    return data;
+  } catch (e) {
+    return null;
+  }
+}
+
+function setCache(data) {
+  try {
+    wx.setStorageSync(CACHE_KEY, data);
+  } catch (e) {
+    console.warn('cache write failed', e);
+  }
+}
+
+function isCacheValid(cache, base) {
+  if (!cache || !cache.rates || !cache.timestamp) return false;
+  if (cache.base !== base) return false;
+  return Date.now() - cache.timestamp < CACHE_TTL;
+}
+
+function fetchRates(base) {
+  return new Promise((resolve, reject) => {
+    wx.request({
+      url: `https://open.er-api.com/v6/latest/${base}`,
+      method: 'GET',
+      success(res) {
+        if (res.statusCode === 200 && res.data && res.data.result === 'success') {
+          resolve(res.data.rates);
+        } else {
+          reject(new Error('API 返回异常'));
+        }
+      },
+      fail(err) {
+        reject(err);
+      },
+    });
+  });
+}
+
+async function getRates(base) {
+  const cache = getCache();
+
+  if (isCacheValid(cache, base)) {
+    return {
+      rates: cache.rates,
+      updatedAt: cache.updatedAt,
+      fromCache: true,
+    };
+  }
+
+  try {
+    const rates = await fetchRates(base);
+    const now = new Date();
+    const cacheData = {
+      base,
+      rates,
+      timestamp: now.getTime(),
+      updatedAt: formatTime(now),
+    };
+    setCache(cacheData);
+    return {
+      rates,
+      updatedAt: cacheData.updatedAt,
+      fromCache: false,
+    };
+  } catch (e) {
+    // API 失败时用过期缓存兜底
+    if (cache && cache.rates) {
+      return {
+        rates: cache.rates,
+        updatedAt: cache.updatedAt + '（缓存）',
+        fromCache: true,
+      };
+    }
+    throw e;
+  }
+}
+
+function convert(amount, fromCode, toCode, rates) {
+  if (!rates || !rates[fromCode] || !rates[toCode]) return 0;
+  if (fromCode === toCode) return amount;
+  return (amount / rates[fromCode]) * rates[toCode];
+}
+
+function formatAmount(value, code) {
+  // 日元、韩元、越南盾、印尼盾等无小数货币
+  const noDecimal = ['JPY', 'KRW', 'VND', 'IDR'];
+  if (noDecimal.includes(code)) {
+    return Math.round(value).toLocaleString('en-US');
+  }
+  return Number(value.toFixed(2)).toLocaleString('en-US', {
+    minimumFractionDigits: 2,
+    maximumFractionDigits: 2,
+  });
+}
+
+function formatRate(value) {
+  if (value >= 100) return value.toFixed(2);
+  if (value >= 1) return value.toFixed(4);
+  return value.toFixed(6);
+}
+
+function formatTime(date) {
+  const y = date.getFullYear();
+  const m = String(date.getMonth() + 1).padStart(2, '0');
+  const d = String(date.getDate()).padStart(2, '0');
+  const h = String(date.getHours()).padStart(2, '0');
+  const min = String(date.getMinutes()).padStart(2, '0');
+  return `${y}-${m}-${d} ${h}:${min}`;
+}
+
+module.exports = {
+  getRates,
+  convert,
+  formatAmount,
+  formatRate,
+};