fix: Resolve onboarding UI overflow and migrate to Dart 3

[DevAnuragT]

Fixes #264

Describe the changes you have made in this PR -

• Fixed UI Overflow on Onboarding Screen:

  • Removed the hardcoded height: 250 constraint on the text container in onboard_page.dart which was causing RenderFlex overflow errors.
  • Wrapped the text description row in an Expanded widget in onboarding.dart to ensure it takes up remaining space gracefully instead of overflowing.
  • Added a regression test (test/onboarding_widget_test.dart) to verify the fix.

• Dart 3 & Flutter 3.x Migration:

  • Updated the project to support Dart 3 (Null Safety enforcement).
  • Replaced the deprecated uni_links package with app_links to fix build failures on modern Android/iOS versions.
  • Migrated deprecated Material theme properties (e.g., bodyText1 → bodyLarge, accentColor → colorScheme.secondary).
  • Updated Gradle configuration and dependencies in pubspec.yaml to ensure the app builds successfully on the latest Flutter stable channel.