implement clearer UX for transparent color in color picker

[junnjiee]

Context

Fixes #4081

Implementation

Instead of an X symbol, a transparent color icon is used to hint that the button removes the color.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
nusmods-export	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 21, 2025 11:12am

[vercel]

@junnjiee is attempting to deploy a commit to the modsbot's projects Team on Vercel.

A member of the Team first needs to authorize it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.01%. Comparing base (988c6fd) to head (f85e2ed).
Report is 124 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4111      +/-   ##
==========================================
- Coverage   54.52%   53.01%   -1.51%     
==========================================
  Files         274      288      +14     
  Lines        6076     6675     +599     
  Branches     1455     1632     +177     
==========================================
+ Hits         3313     3539     +226     
- Misses       2763     3136     +373     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.