fix: update color when picker drag ends outside the selection box

[MOULINIER]

(First commit to a major project, sorry if I made any mistakes on the PR)

Summary
When picking a color on the color picker box, it bugged me that when selecting colors at the corners like #FFFFFF or #000000 they weren't actually selected. It seems that when the cursor ends up outside of the box when selecting, the color isn't registered.

Changes

• Use react-colorful built-in onChangeEnd (added recently in 5.7.0) to mitigate the issue, as the feature was made for issues like this (onMouseUp not working if you drag outside of the component omgovich/react-colorful#201)
• Removed the handlePointerUp function / useCallback import

Tests and AI
I ran the integrated bun run --filter ui test.
Tested the fix manually.

I used AI to help me debug and learn the structure (I am a CS student and still learning!)

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