Create sandbox package for testing w3ux library components from source

[Copilot]

This PR implements a new sandbox package that enables real-time testing of w3ux library components and utilities directly from their source files without requiring compilation or publishing to npm.

What's Added

🏗️ Sandbox Package Structure

• Vite + React + TypeScript: Modern development environment with hot module replacement
• Workspace Integration: Added to pnpm workspace configuration for seamless monorepo integration
• Source-only Imports: Direct path mappings to library source files (../library/*/src)

🔧 Configuration

• TypeScript Path Mappings: All @w3ux/* packages mapped to their source directories
• Vite Alias Resolution: Configured to resolve library imports at build time
• ESLint Integration: Properly excluded from main workspace linting to avoid conflicts

📦 Library Integration

The sandbox demonstrates importing and using multiple w3ux packages:

• @w3ux/react-polkicon - Polkadot identicon component
• @w3ux/react-odometer - Animated number display
• @w3ux/utils - Utility functions (address validation, unit conversion, ellipsis formatting)
• @w3ux/hooks - React hooks (useTimeLeft countdown)
• @w3ux/types - TypeScript type definitions

🎯 Usage

# Start development server
cd sandbox && pnpm run dev

# Build for production
pnpm run build

# Lint sandbox code
pnpm run lint

Benefits

• 🚀 Fast Development: Test library changes instantly without compilation
• 🔄 Hot Reload: Changes to library source files trigger immediate updates
• 🧪 Integration Testing: Test multiple packages working together
• 🔒 Type Safety: Full TypeScript support with proper path resolution
• 📝 Documentation: Comprehensive README with usage examples

Example Usage

import { Polkicon } from '@w3ux/react-polkicon'
import { isValidAddress } from '@w3ux/utils'
import { useTimeLeft } from '@w3ux/hooks'

function TestComponent() {
  const address = '5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY'
  const { timeleft } = useTimeLeft()
  
  return (
    <div>
      <Polkicon address={address} />
      <p>Valid: {isValidAddress(address) ? 'Yes' : 'No'}</p>
      <p>Time: {timeleft.raw.minutes}m {timeleft.raw.seconds}s</p>
    </div>
  )
}

This enables testing observables and react-connect-kit modules directly within w3ux as requested, providing a robust development environment for library integration testing.

Fixes #306.

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