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/assets/config.json

@@ -1,7 +1,7 @@
 {
-  "appName": "SilverDesk Staging",
+  "appName": "SilverSHELL AppStore",
   "appVersion": "1.0.0",
-  "targetUrl": "https://silverdesk-staging.silverlabs.uk/",
+  "targetUrl": "https://store.silverlabs.uk",
   "showUrlBar": false,
   "allowNavigation": true
 }