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>

project.config.json

@@ -1,6 +1,6 @@
 {
   "miniprogramRoot": "miniprogram/",
-  "appid": "",
+  "appid": "wx7401eb1b9415ae4d",
   "projectname": "currency-converter",
   "description": "汇率换算工具",
   "setting": {
@@ -26,8 +26,29 @@
       "ignore": [],
       "disablePlugins": [],
       "outputPath": ""
-    }
+    },
+    "compileWorklet": false,
+    "uglifyFileName": false,
+    "packNpmManually": false,
+    "packNpmRelationList": [],
+    "minifyWXSS": true,
+    "minifyWXML": true,
+    "localPlugins": false,
+    "disableUseStrict": false,
+    "useCompilerPlugins": false,
+    "condition": true,
+    "swc": false,
+    "disableSWC": true
   },
   "compileType": "miniprogram",
-  "condition": {}
-}
+  "condition": {},
+  "simulatorPluginLibVersion": {
+    "wxext14566970e7e9f62": "2.27.3"
+  },
+  "packOptions": {
+    "ignore": [],
+    "include": []
+  },
+  "editorSetting": {},
+  "projectArchitecture": "multiPlatform"
+}