feat(appstore): add AppStore client features

- AppStoreJsBridge: JS bridge for native APK install, version check, update
- InstalledAppDao, InstalledApp entity: Room DB for tracking installed apps
- UpdateCheckerWorker: background update polling via WorkManager
- InstallerService: APK download + SHA-256 verify + PackageInstaller session
- Updated AndroidManifest: REQUEST_INSTALL_PACKAGES, FileProvider, receivers
- Updated MainActivity: notification channel, WorkManager, JS bridge wiring
- config.json: targetUrl = https://store.silverlabs.uk
- file_paths.xml: FileProvider paths for APK installs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/src/main/kotlin/uk/silverlabs/silverdroid/update/UpdateCheckResponseDto.kt

@@ -0,0 +1,12 @@
+package uk.silverlabs.silverdroid.update
+
+import kotlinx.serialization.Serializable
+
+@Serializable
+data class UpdateCheckResponseDto(
+    val hasUpdate: Boolean,
+    val latestVersion: String,
+    val downloadUrl: String? = null,
+    val sha256: String? = null,
+    val releaseNotes: String? = null
+)