fix: align sync engine mapper with IndexedDB store schema #4091

Originally written by @ccantynz-alt on GitHub.
Imported from https://github.com/ccantynz-alt/AlecRae.com/pull/40

Update mapMessageToCachedEmail to handle optional name fields correctly via shared mapContact helper.

https://claude.ai/code/session_01Jiu6ukfSLoBm4F5RVz5YBx

Description

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no functional changes)
• Documentation update
• Infrastructure / CI change

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Checklist

• My code follows the project style guidelines
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or my feature works
• New and existing tests pass locally (bun run test)
• Lint and type-check pass (bun run lint && bun run typecheck)
• I have updated documentation where necessary

Breaking changes

N/A

Screenshots

N/A