Migrate project to Flutter 3.11+ with null safety and modern tooling

[KumarSunil17]

Bring the UI kit up to current Flutter/Dart so it builds and runs on
modern SDKs without legacy pre-null-safety dependencies.
Dependencies and SDK

• Require Dart ^3.11.0 and refresh pub packages (http, rxdart,
  font_awesome_flutter, cupertino_icons)
• Replace unmaintained flutter_masked_text with mask_text_input_formatter
• Remove unused random_pk; add flutter_lints and analysis_options.yaml
  Platform and build
• Regenerate Android/iOS (and add web, desktop) project scaffolding
• Move Android to Gradle 8.14, Kotlin MainActivity, and AndroidX
• Update AndroidManifest for embedding v2; drop legacy Groovy Gradle files
  Application code
• Enable null safety across models, services, blocs, and UI
• Update deprecated Material APIs (buttons, themes, inherited widgets,
  ScaffoldMessenger, Uri-based HTTP)
• Rewrite credit card input with MaskTextInputFormatter
• Fix login bloc loginSink wiring and api subscription lifecycle
  Tests
• Replace counter smoke test with a basic app launch test