feat(android): implement split tunneling (per-app proxy) UI

[SteeaaN]

Related Issue

Closes #27

Summary

This PR introduces the UI and state management for the Android-specific Split Tunneling (Per-App Proxy) feature.

Depends on the native core PR: TrustTunnel/TrustTunnelClient#79

Changes

• UI: Created a new PerAppProxyScreen to display and filter installed Android apps using the installed_apps plugin.
• Platform Check: The "Per-app proxy" menu item in settings is strictly visible only on Android to prevent crashes or confusing UX on iOS/macOS.
• Storage: Added SharedPreferences logic in SettingsDataSource to persist the chosen per-app routing preferences.
• Integration: Modified VpnDataSource and configuration_codec to inject the routing preferences into the Tun plugin model before starting the tunnel.
• UX: Added a dynamic "Restart required" banner that prompts the user to apply changes if the VPN is currently active.

Checklist

• Code generation is up to date (make init)
• Dart analysis passes with no issues
• Documentation updated (if applicable)
• No secrets or credentials committed