chore: revise the signup with phone number screen to match the design

[lkmandy]

Description

• Revise the signup with phone number screen to match the actual design on Figma

Before

After

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ 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.