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>

app/src/main/AndroidManifest.xml

@@ -11,6 +11,7 @@
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
+    <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />
 
     <application
         android:name=".A2iApp"
@@ -45,5 +46,15 @@
             </intent-filter>
         </service>
 
+        <provider
+            android:name="androidx.core.content.FileProvider"
+            android:authorities="${applicationId}.fileprovider"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/file_paths" />
+        </provider>
+
     </application>
 </manifest>