style: use fluorescent yellow for highlighter, separate from color-picker yellow

[j4rviscmd]

Summary

• Separate the highlighter button from the colour-picker's "yellow" by using a dedicated "highlight" style value instead of the built-in "yellow"
• Change the highlighter colour from a deep mustard (oklch 0.92 0.17 90) to a softer fluorescent yellow (oklch 0.96 0.11 95) in light mode
• Adjust dark-mode highlighter to a lighter, more yellow shade (oklch 0.75 0.10 95)
• Restore the colour-picker's yellow to the original mustard shade so the two features are visually distinct

Test plan

• Click the highlighter button (fluorescent pen icon) and verify it applies a soft fluorescent yellow
• Use the colour-picker dropdown → Background → yellow and verify it applies the deeper mustard shade
• Verify both colours look correct in light and dark modes
• Verify toggling the highlighter on/off works correctly
• Verify the highlighter button shows active state only for the fluorescent highlight, not for colour-picker yellow

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