fix: onboarding screens duplicating

Description

refactored onboarding screens code from PageView.builder() to a normal PageView()

[ ] ✨ New feature (non-breaking change which adds functionality)
[x] 🛠️ Bug fix (non-breaking change which fixes an issue)
[ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
[x] 🧹 Code refactor
[ ] ✅ Build configuration change
[ ] 📝 Documentation
[x] 🗑️ Chore

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[ ] I listed at least one issue that this PR fixes in the description above.
[ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy,
[ ] I updated/added relevant documentation (doc comments with ///).
[ ] I added new tests to check the change I am making, or this PR is [test-exempt].
[ ] All existing and new tests are passing.