Add `.palette()` mode to `field.color()` for brand-constrained pickers

[akladekouassi]

Summary

Extend field.color() (see issue 01) with a .palette(brandTokens) modifier that constrains the picker to a fixed color palette. Free hex remains available as an explicit opt-in via .allowCustom(true).

Why this would help

• Design-system-aware forms, theme editors, label / category selectors that must stay on-brand.
• Today users either use field.select() with hard-coded options (and lose the visual swatches) or field.color() and police the value at submit time.
• A palette-constrained picker is a one-line change once the base color field exists.

Proposed API

field.color('Label color')
  .palette(['#0f6fdc', '#22c55e', '#f59e0b', '#ef4444'])
  .allowCustom(false);

Suggested implementation areas

• src/core/field-descriptors/color/ColorFieldBuilder.ts — depends on issue 01
• a palette validator that rejects values outside the configured set when allowCustom(false)
• src/renderers/web/ and src/renderers/native/ — swatch grid + optional free-form input toggle

Acceptance Criteria

• Values outside the palette are rejected when allowCustom(false)
• Toggling allowCustom(true) reopens the standard color picker as a secondary path
• Tests for both modes and the validator rejection

[github-actions]

Mirrored to monorepo issue: https://github.com/runilib/runilib/issues/187