[TRIVIAL] Add '/ui' route to testapp + update button colors

[tdrach]

Summary

This PR introduces two updates:

1. A UI playground for SIWB & BP buttons at /ui route in the testapp
2. Updating of colors to match Figma and system requirements for all interactions

Full breakdown of changes...

🎨 Color System Architecture

• 32 total color variables with consistent naming convention
• 24 variables for Sign In buttons: BUTTON_{LIGHT|DARK}_{SOLID|TRANSPARENT}_{STATE}_{PROPERTY}
• 8 variables for Base Pay buttons: BASEPAY_{LIGHT|DARK}_{STATE}
• Separate namespaces prevent cross-contamination between button types

🔄 Sign In Button Color Inversion

• Light mode: White background (#FFFFFF) with black text (#000000) and subtle border (#EEF0F3)
• Dark mode: Black background (#000000) with white text (#FFFFFF) and transparent border
• Improved contrast and modern aesthetic

🎯 Interaction Simplification

• Removed transform: translateY(-1px) effects
• Removed box-shadow animations
• Clean color-only transitions for better performance and accessibility

🌙 System Theme Integration

• Added /ui test route on the testapp with system theme detection
• Dynamic background colors: #FFFFFF (light) / #000000 (dark)
• Comprehensive button showcase across different backgrounds

🧪 Test Coverage

• Updated all test files to match new color values
• Comprehensive coverage for all button variants and states
• All tests passing ✅

📁 Files Changed

• colors.ts: New comprehensive color variable system
• Button components: Updated to use new color variables
• Test route: New /ui page for visual testing
• All tests: Updated expectations to match new colors

How did you test your changes?

✅ Manual Testing

• Tested all button variants in light/dark modes
• Verified system theme detection works correctly
• Tested hover/active states across different backgrounds
• Confirmed color consistency across all frameworks (React, Vue, Svelte wrap Preact)

✅ Automated Testing

• All existing tests updated and passing
• Test coverage for new color variables
• Validated button rendering in different color schemes

✅ Visual Testing

• Created comprehensive test page at /ui route
• Showcased buttons on multiple background colors
• Verified accessibility and contrast ratios

The new design system provides a solid foundation for consistent button styling across the entire SDK while maintaining backwards compatibility and improving the overall user experience.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2