fix: Onboarding next button event, signup and login design fix

[DesmondTambe]

Description

• Onboarding next screen onPress event handler for onboarding slide animation
• Login design fixes ( ColorScheme, spacing around widget, on-page screen event)
• Signup fixes relative to Figma design

Type of Change

• [ ] ✨ 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)
• [ ] 🧹 Code refactor
• [ ] ✅ Build configuration change
• [ ] 📝 Documentation
• [ ] 🗑️ Chore

Pre-launch Checklist

• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] I read and ran all relevant commands as specififed in the Running Tests section of the Contributor Guide.
• [ ] The title of the PR follows the Conventional Commits guideline
• [ ] My local branch follows the naming standards in the Deepsource Branch Naming Convention or Biodiversity Branch Naming Convention
• [ ] 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.

[jeffrey0606]

You bro please also resolve this one so I can merge it

[DesmondTambe]

Yoo @jeffrey0606

[jeffrey0606]

Sorry 😅 @DesmondTambe forgot to switch branch on my end 🥲