🔨 Split ColorUtils into ColorParser, ColorConversion, and ColorAccessibility

[guiyanakuang]

Closes #3940

Summary

• Split monolithic ColorUtils.kt (695 lines, 42 functions) into 3 focused modules following the Single Responsibility Principle:
  • ColorParser — CSS color string parsing (hex, RGB, RGBA, HSL, named colors)
  • ColorConversion — Color space conversions and formatting (RGB↔HSV, hex/RGB string output)
  • ColorAccessibility — Contrast detection, dark/light analysis, color lightening (WCAG)
• Removed ~8 unused public functions (~120 lines of dead code)
• Updated all 13 caller files and 2 test files with new imports

Test plan

• ./gradlew ktlintFormat passes
• ./gradlew app:desktopTest — all 209+ tests pass
• Manual verification of color-related UI (side preview, color paste, HTML/RTF views)