feat(config): backup v1 file before v2 migration

- Add timestamped backup at `~/.cc-switch/config.v1.backup.<ts>.json`
- Keep provider files untouched; only cc-switch metadata is backed up
- Remove UI notification plan; backup only as requested
- Update CHANGELOG with migration backup notes

src/lib/tauri-api.ts

@@ -167,6 +167,8 @@ export const tauriAPI = {
     }
   },
 
+  // （保留空位，取消迁移提示）
+
   // 选择配置文件（Tauri 暂不实现，保留接口兼容性）
   selectConfigFile: async (): Promise<string | null> => {
     console.warn("selectConfigFile 在 Tauri 版本中暂不支持");