Enhance the web index html from parsec flutter

[Victorcorcos]

Summary

This PR adds a comprehensive web example interface that demonstrates the parsec_web WebAssembly integration with a polished user experience with improved loading UI.

Key Features

• Enhanced Loading Experience: Custom loading spinner with progress indicator showing "Loading Parsec..." and "Initializing WebAssembly"
• Modern UI Design: Gradient background with clean, responsive styling using system fonts
• WebAssembly Integration: Proper ES6 module loading for parsec-web JavaScript wrapper
• Flutter Bootstrap: Optimized Flutter web initialization sequence with proper lifecycle management
• Error Handling: Graceful fallback when WASM modules are unavailable with console warnings

Technical Implementation

• Uses <script type="module"> for parsec-web JavaScript wrapper to support ES6 exports
• Implements proper WASM initialization checking (window.parsecWebReady) before Flutter app startup
• Provides visual feedback during both WebAssembly and Flutter engine loading phases
• Follows Flutter web best practices for production deployments with proper service worker handling
• Dynamic WASM module imports handled by the wrapper during initialization

File Structure

parsec/example/web/index.html
├── Meta tags and iOS app configuration
├── Custom CSS with loading spinner animation
├── Parsec WebAssembly integration scripts
└── Flutter web bootstrap with proper initialization

Test Plan

To verify this enhancement works correctly:

1. Generate WASM files: cd parsec_web && dart bin/generate.dart
2. Launch web demo: cd parsec/example && flutter run -d chrome
3. Verify loading sequence:
   • Loading spinner should appear immediately
   • "Loading Parsec..." and "Initializing WebAssembly" text visible
   • Console should show either "✅ Parsec WebAssembly library loaded successfully" or "⚠️ Parsec WebAssembly library not found"
   • Loading indicator disappears when Flutter app starts
4. Test responsiveness: Resize browser window to verify responsive design
5. Verify WebAssembly integration: Use parsec evaluation functions in the demo

Dependencies

This enhancement depends on the parsec_web package structure with:

• packages/parsec_web/parsec-web/js/equations_parser_wrapper.js (ES6 module)
• WASM files generated by dart bin/generate.dart

Breaking Changes

None. This is a new file that enhances the existing web example without affecting other platforms or core functionality.