fix: Issues with scheduling timezone change without end time

[nayan-bagale]

What does this PR do?

• Fixes Broken Ui in dark mode
• Fixes Add button always stays disabled.
• Fixes [CAL-5127] Issues with scheduling timezone change without end time #19095 (GitHub issue number)
• Fixes CAL-5127 (Linear issue number - should be visible at the bottom of the GitHub issue description)

Before:
Screencast from 2025-02-05 01-53-37.webm

After:
Screencast from 2025-02-05 01-57-22.webm

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

[vercel]

@nayan-bagale is attempting to deploy a commit to the cal-staging Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

All committers have signed the CLA.

[graphite-app]

Graphite Automations

"Add consumer team as reviewer" took an action on this PR • (02/04/25)

1 reviewer was added to this PR based on Keith Williams's automation.

"Add community label" took an action on this PR • (02/04/25)

1 label was added to this PR based on Keith Williams's automation.

[nayan-bagale]

Hey @CarinaWolli , I’ve changed the DatePicker code and added the Calendar.tsx component to handle single-day selection. Since Calendar.tsx is now being reused, should I remove the test cases for DatePicker?