feat: add highlighter marker button to formatting toolbar

[j4rviscmd]

Summary

• Add a yellow highlight toggle button to the text selection bubble menu
• Button is positioned after the built-in color-style button in the formatting toolbar
• Uses BlockNote's existing backgroundColor style with custom CSS for enhanced visibility
• Light mode: vibrant yellow background with subtle padding and border-radius
• Dark mode: darker amber background with adjusted text color for readability

Test plan

• Select text in the editor and verify the highlight button appears in the bubble menu
• Click the highlight button and verify yellow highlight is applied
• Click again to toggle off the highlight
• Verify highlight appearance in both light and dark themes
• Verify the highlight persists after saving and reloading a note
• Verify the existing color picker button still works alongside the highlight button

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