Update litmus-cli to load new theme format (litmus-dv2l)

Migrate litmus-cli from old Theme struct to ThemeDefinition + ProviderColors:

• Load ThemeDefinition + pick one ProviderColors (first available, or –provider flag)
• Thread ProviderColors through to rendering (swatches, mockups, live views)
• Minimal changes — CLI is simpler than web

Depends on: new model types, converted themes

Plan

1. Update theme_data.rs:

   • Add load_bundled_provider_themes(provider: Option<&str>) -> Vec<Theme>
   • Uses load_themes_dir() to load ThemeDefinitions + ProviderColors
   • For each ThemeDefinition, picks first available ProviderColors (or filtered by provider)
   • Converts (ThemeDefinition, ProviderColors) → Theme via to_theme() helper
   • Falls back to hardcoded themes if nothing found

2. Update main.rs:

   • Add --provider CLI flag
   • Pass to load_bundled_provider_themes()

3. Keep all widget code unchanged — still render with &Theme

Todo

• Add ProviderColors → Theme conversion
• Update load_bundled_themes to use load_themes_dir
• Add –provider CLI flag
• Tests pass, zero warnings
• Review

Summary of Changes

Migrated litmus-cli from directly loading Theme objects to using the new provider-based system:

• ProviderColors::to_theme(): New method converts provider colors + theme name into a renderable Theme
• load_bundled_themes(provider): Rewrote to use load_themes_dir(), picking one ProviderColors per ThemeDefinition (first available alphabetically, or filtered by –provider)
• –provider CLI flag: Simple arg parser for –provider filtering
• All widget rendering unchanged — still receives &Theme
• Hardcoded fallback themes preserved for environments without themes/ directory
• 160 tests pass, zero warnings