fix: rsvp controls

[wrycu]

Summary

Introduces canRSVP as a concept independent of event/calendar edit-ability (since you can typically edit your own calendar). Updates existing calls to leverage canRSVP where appropriate while retaining existing functionality (which was a bit of a pain). Updates the rendering to leverage RSVP controls prior to edit-ability checks, and adds tests for this going forward.

Changes

• Adds canUserRSVP() to /lib/calendar-editability.ts to determine if a user can RSVP
  • This is added separately as a received invite typically lands on a calendar where edits are permitted
• Moves RSVP controls rendering to leverage new data point
• Adds tests for RSVP controls to prevent this from happening again

Related issues

Closes #937

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)
• Documentation update
• Refactor / code quality improvement
• Chore / dependency update / CI change

Checklist

• I have read the Contributing Guide
• My code follows the project's code style and conventions
• I have run npm run typecheck && npm run lint and there are no errors
• The build passes (npm run build)
• I have tested my changes locally
• I have added or updated documentation if needed
• I have updated translations (locales/) if my changes affect user-facing text
• I have included screenshots or a screen recording for UI changes

Screenshots / demo

N/A

Notes for reviewers

Existing integration tests in Docker do not check for this functionality (which is probably why it was able to happen). I did not add tests there.