fix: add ios update check,remove boilerplate platform interface,rewri…

[akshaynexus]

Closes #16, #17

Summary

Adds iOS update checks and a shared update API for iOS and Android.

Changes

• Added iOS update checking via iTunes Lookup API (pure Dart, no native code)
• Compares installed vs App Store version using pub_semver
• Added unified cross-platform API:
  • checkUpdate() — returns AppUpdateInfo with platform-specific details
  • startUpdate() — presents App Store page on iOS, starts Play Core update on Android
  • checkAndUpdate() — convenience one-liner that checks and starts the update if available
• Added UpdateConfig to set defaults (appStoreId, iosAppStoreRegion, androidUpdateType) so values aren't repeated per call
• Added AndroidUpdateType enum (immediate / flexible) for configuring default Android update behavior
• Added models: AppUpdateInfo, AppUpdateInfoIos, UpdateConfig
• Removed platform-interface boilerplate (InAppUpdateFlutterPlatform, plugin_platform_interface)
• Added dependencies: package_info_plus, pub_semver
• Updated README with setup for iOS-only, Android-only, and both-platforms scenarios
• Added AGENTS.md and CLAUDE.md

Test status

13/13 tests passing

[akshaynexus]

Should be ready for review/tests now