Quetzal 0.6.3

• App 自己更新：安卓 App 每次打开界面（启动、从后台回来）都问 GitHub 有没有新的正式版（正在检查时不重复），有就在顶部提示「Quetzal App 有新版本」；控制 → 服务 → Quetzal App 一节可随时「检查新版本」，「下载并安装」一键完成：下载 quetzal-<版本>-android-arm64.apk 到缓存目录（进度条）、核对 SHA256SUMS、交给系统安装器（首次先带去系统设置允许 Quetzal 安装应用，回来自动接着装）；连不上 GitHub 时「去下载页」手动下载。装好的新 App 打开后，发现内置的运行基座比运行中的新，直接进安装向导的升级一步，两层升级都不用再找 APK。原生新增 MethodChannel quetzal/updater 与 FileProvider（只共享缓存目录），清单声明 REQUEST_INSTALL_PACKAGES；Dart 侧新依赖 crypto（核对 SHA256，原本已是间接依赖）。
• Self-updating app: every time the Android app comes to the foreground (launch or return from the background) it asks GitHub for the latest release (never two checks at once) and shows "A new Quetzal app version is available" at the top; the new Control → Service → Quetzal App section can check for updates any time and download and install in one tap: it downloads quetzal-<version>-android-arm64.apk into the cache directory (progress bar), checks it against SHA256SUMS and hands it to the system installer (the first time it opens the system settings so you can allow Quetzal to install apps; installation continues when you come back); when GitHub is unreachable, Download page lets you fetch the APK by hand. When the new app opens and finds its bundled runtime newer than the running one, it goes straight to the upgrade step of the setup wizard, so neither layer needs hunting for an APK any more. Native side: a new MethodChannel quetzal/updater and a FileProvider (cache directory only); the manifest declares REQUEST_INSTALL_PACKAGES; Dart gains the crypto dependency (SHA256; it was already a transitive dependency).

Full Changelog: v0.6.2...v0.6.3