fix: replace sslcommerz_flutter (non-existent) with WebView approach

- Remove sslcommerz_flutter (not on pub.dev) from pubspec
- Add webview_flutter ^4.10.0 for in-app SSLCommerz payment WebView
- New SSLCommerzWebViewScreen: opens gateway URL, intercepts /ssl-callback navigation
- plans_screen.dart: SSLCommerz flow now uses WebView + verify by tran_id
- flutter pub get verified successfully

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