Add optional Theme Preview generation

[Claude]

Adds opt-in generation of a Theme+Preview.swift file containing a SwiftUI view that renders all theme tokens as a visual palette.

Changes

• JSON Schema: Added shouldGeneratePreview boolean flag to config section (defaults to false)
• ThemeFile.GenerationConfig: Added shouldGeneratePreview property with convenience accessor on ThemeFile
• ThemePreviewGenerator: New generator producing preview file with:
  • Color swatches in adaptive grid
  • Gradient strips with linear gradient visualization
  • Shadow showcase cards with proper shadow enum handling
  • Private helper components (ColorSwatch, GradientStrip, ShadowCard, ShadowModifier)
• ThemeFileGenerator: Conditionally includes preview file when flag is enabled
• Test coverage: 21 unit tests for generator, 4 integration tests for flag behavior
• Documentation: Updated README with usage example and feature description

Usage

Enable in theme.json:

{
  "styles": {
    "colors": ["surface", "primary"],
    "gradients": ["hero"]
  },
  "config": {
    "outputPath": ".",
    "shouldGeneratePreview": true
  }
}

Generated preview resolves tokens through @Environment(\.theme) and @Environment(\.self), adapting to color scheme and environment automatically:

public struct ThemePreview: View {
    @Environment(\.theme) var theme
    @Environment(\.self) var environment
    
    public var body: some View {
        ScrollView {
            VStack(alignment: .leading, spacing: 24) {
                // Color swatches, gradient strips, shadow cards
            }
        }
    }
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Optionally generate Theme Preview</issue_title>
<issue_description>Generate a Theme+Preview.swift file containing a SwiftUI view that renders all tokens as a visual palette, something like:

public struct ThemePreview: View {
      @Environment(\.theme) var theme

      public var body: some View {
          ScrollView {
              // Color swatches grid
              // Gradient strips
              // Mesh Gradient rects
              // Shadow showcase cards
          }
      }
  }

  #Preview { ThemePreview() }

Could be opt-in via shouldGeneratePreview config flag.</issue_description>

Comments on the Issue (you are @claude[agent] in this section)

@rozd @claude work on it

• Fixes Optionally generate Theme Preview #8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.46%. Comparing base (f168dc0) to head (0663c99).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
+ Coverage   99.35%   99.46%   +0.10%     
==========================================
  Files          30       32       +2     
  Lines        1405     1677     +272     
==========================================
+ Hits         1396     1668     +272     
  Misses          9        9              

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