chore: stabilize engine, enable modern Dart features, and safeguard publication

[jaypal1046]

📝 Description

This PR stabilizes the FlutterJS engine by standardizing the SDK version and fixing workspace resolution. It also enables support for modern Dart features like Dot Shorthands and ensures all packages are protected from accidental publication.

🏗️ Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🎨 Code style update (formatting, local variables)
• 🔨 Refactoring (no functional changes, no api changes)
• 🧪 Tests
• ⚙️ CI/CD

📋 Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes

🧪 How to Test

1. Run dart pub get in the root directory.
2. Verify that all dependencies are resolved without errors.
3. Open examples/counter/lib/main.dart and verify the .fromSeed syntax is recognized correctly (requires Dart 3.10+).