feat: overhaul plugin UI/UX and update JSON export schema

[findyourexit]

UI/UX Overhaul Inc. JSON Schema Update (DTCG Spec. Compliance)

Description

Overview

This PR represents a major overhaul of FigGit, introducing a completely redesigned user interface with DTCG-compliant schema support, migration from React to Preact, and significant UX improvements. The changes span architectural improvements, bug fixes, and enhanced documentation.

What's Changed

Architecture & Dependencies

• Migrated from React to Preact for improved performance and smaller bundle size
• Integrated @create-figma-plugin/ui for native Figma Desktop styling and automatic theme support
• Updated JSON schema to fully adhere to DTCG specification v2025.10

UI/UX Improvements

• Complete UI restructure with new tab-based layout:
  • Settings Tab: One-time configuration (repository, token, advanced options)
  • Export Tab: Primary workflow with action buttons and status summary
  • Preview Tab: JSON viewer and diff viewer for reviewing changes
• Auto-export on plugin open to reduce time-to-commit steps
• Enhanced diff viewer with syntax highlighting and collapsible sections
• JSON viewer with copy functionality for easy token inspection
• Fixed upstream Banner component icon vertical alignment issue

Bug Fixes

• Fixed stale "View Diff" state caused by remote data cache persisting after commits
• Improved state management in PluginContext to properly clear cached data

Documentation

• Added FigGit logo to README.md
• Updated README.md with comprehensive feature list and installation instructions
• Updated coverage badge (94.51%)
• Enhanced documentation for DTCG compliance (namely the schema example)

Testing

• Added comprehensive test coverage for new utilities
• Removed obsolete component and hook tests
• Improved test configuration with updated Vitest setup

New Components & Utilities

New Components:

• ActionButtons.tsx - Export and commit action controls
• StatusSummary.tsx - Export status visualization
• DiffViewer.tsx - Token-level change visualization
• JsonViewer.tsx - Formatted JSON display with copy
• TabLayout.tsx - Consistent tab-based navigation
• Disclosure components for settings organization

New Utilities:

• colorUtils.ts - DTCG-compliant color format conversion
• dtcgUtils.ts - DTCG schema validation and transformation
• Enhanced buildDtcgJson.ts - Full DTCG export implementation

Notes

• This PR sets the foundation for future enhancements
• The modular component architecture makes it easier to add new features
• DTCG compliance ensures compatibility with industry-standard design token tools