feat: show active state on color toolbar button

[j4rviscmd]

Summary

• Replace BlockNote's default ColorStyleButton with a custom CustomColorStyleButton that visually indicates when a text or background color is active
• When a color is applied, the toolbar button shows the same accent background as other active commands (bold, italic, etc.)
• Exclude the highlighter style (backgroundColor: "highlight") from the color button's active detection so the two features don't interfere
• Match the Toggle button padding for consistent toolbar sizing

Test plan

• Select text and apply a text color → color button shows active accent background
• Select text and apply a background color → color button shows active accent background
• Apply highlighter (fluorescent yellow) → color button does NOT show active state
• Remove all colors → color button returns to inactive state
• Color picker dropdown still works correctly (text and background color selection)
• Button width matches other toolbar buttons

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