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/java/com/a2i/forwarder/A2iApp.kt

@@ -7,6 +7,7 @@ import com.a2i.forwarder.core.ConnectivityMonitor
 import com.a2i.forwarder.core.LogStore
 import com.a2i.forwarder.core.PhoneCallMonitor
 import com.a2i.forwarder.core.SmsFallbackForwarder
+import com.a2i.forwarder.core.UpdateChecker
 import com.a2i.forwarder.store.AppRulesStore
 import com.a2i.forwarder.store.SettingsStore
 import kotlinx.coroutines.CoroutineScope
@@ -30,6 +31,8 @@ class A2iApp : Application() {
         private set
     lateinit var smsForwarder: SmsFallbackForwarder
         private set
+    lateinit var updateChecker: UpdateChecker
+        private set
 
     override fun onCreate() {
         super.onCreate()
@@ -43,6 +46,8 @@ class A2iApp : Application() {
         connectivityMonitor.start()
         carrierBalanceQuery = CarrierBalanceQuery(this)
         smsForwarder = SmsFallbackForwarder(this)
+        updateChecker = UpdateChecker(this)
+        updateChecker.start()
     }
 
     companion object {