feat: Add hexadecimal color input to color picker (fixes #3381)

[majiayu000]

Summary

Add a hexadecimal input field to the color picker component, allowing users to input colors using hex notation (e.g., #FF5500) in addition to the visual palette.

Changes

• Add satus.color.rgbToHex() function for RGB to hex conversion
• Add satus.color.hexToRgb() function for hex to RGB conversion
  • Supports both full (#RRGGBB) and shorthand (#RGB) notation
• Add hexadecimal input field to color picker modal
  • Input syncs with palette, hue slider, and color preview
  • Palette and hue slider update hex input when changed
• Add CSS styles for hex input container, label, and input field
• Add unit tests for color conversion and UI components

Features

• HEX input field appears below the hue slider in the color picker
• Typing a valid hex color updates the palette, cursor position, hue slider, and preview
• Moving the palette cursor or hue slider updates the hex input
• Supports both #RRGGBB and #RGB shorthand formats
• Invalid hex values are ignored (no error shown, just no update)

Test plan

• Open Themes > Custom and click on a color picker
• Verify the HEX input field appears below the hue slider
• Type a valid hex color (e.g., #FF5500) and verify the palette updates
• Verify the color preview updates when typing a valid hex color
• Move the palette cursor and verify the hex input updates
• Move the hue slider and verify the hex input updates
• Verify shorthand hex (#F50) works correctly
• All unit tests pass (npm test)

Fixes #3381

[ImprovedTube]

thanks @majiayu000

tested?
(it can easily be tested. no need to submit prior or submit long LLM messages.)