feat: in-app update check + download + install

Auto-check GitHub Releases latest on launch (24h throttle), show an
update banner on the home screen, download the APK in-app with progress,
and trigger the system installer via FileProvider + REQUEST_INSTALL_PACKAGES.

- core/UpdateChecker: fetch releases/latest, semver compare, stream APK
  to externalFilesDir, install via FileProvider (handles MIUI unknown-sources).
- SettingsStore: updateCheckEnabled / lastUpdateCheckTime / ignoredVersion.
- HomeScreen: update banner + UpdateDialog.
- SettingsScreen: "关于与更新" card with auto-check toggle and manual check.
- manifest: REQUEST_INSTALL_PACKAGES + FileProvider; new res/xml/file_paths.xml.
- version 1.4.2 -> 1.5.0 (versionCode 7 -> 8).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -23,6 +23,7 @@
   - 余额 ≤5 条自动停用，下次打开 App 提醒
   - 每 5 分钟最多发 1 条，避免刷屏
   - 自动解析失败时回退手动额度计数
+- **应用内更新**：每天自动检查 GitHub 新版本，发现新版后一键下载并通过系统安装器升级，无需手动去网页下载。
 
 ## 快速开始