Add custom color palette library with iCloud sync

[nwoodfine]

Feature Request

Add support for user-defined color palettes that appear as swatch bars in the main interface, similar to the color history bar.

Proposed behavior

• Users define palettes in an INI-style text format in Preferences:

  [Brand Colors]
  #FF5733, #33FF57, #3357FF
  
  [Grays]
  rgb(50, 50, 50), rgb(100, 100, 100), hsl(0, 0%, 80%)
  

• Supports all color formats Pika already understands (hex, rgb, hsl, hsb, lab, oklch)
• Colors can have optional labels: #FF5733 (Primary Red)
• Palettes appear as swatch bars below color history
• Tapping a swatch applies it as the foreground color and copies to clipboard
• Palette data syncs across devices via iCloud Key-Value Store
• Limits: up to 5 palettes, 20 colors each
• Validation feedback in the editor (character count, error messages)

Use case

Designers and developers often work with a fixed set of brand/project colors. Having quick access to these in Pika would save time vs. picking or typing them repeatedly.

I have an implementation ready that I'll submit as a PR.