init: 汇率换算工具小程序骨架

- 两个页面：首页（双向换算）+ 汇率总览（一对多）
- 基础项目结构和配置
- AppID 待填入

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

miniprogram/app.json

@@ -0,0 +1,17 @@
+{
+  "pages": [
+    "pages/index/index",
+    "pages/overview/overview"
+  ],
+  "window": {
+    "navigationBarTitleText": "汇率换算",
+    "navigationBarBackgroundColor": "#FFFFFF",
+    "navigationBarTextStyle": "black",
+    "backgroundColor": "#F5F5F5",
+    "backgroundTextStyle": "dark"
+  },
+  "style": "v2",
+  "componentFramework": "glass-easel",
+  "sitemapLocation": "sitemap.json",
+  "lazyCodeLoading": "requiredComponents"
+}