feat(middleware): add page-level feature flag protection via definePageMeta

[nicokempe]

✅ PR Checklist

Please ensure the following before submitting your PR:

• My commit message follows Conventional Commits
• I’ve tested the changes and confirmed they work as expected
• I’ve updated any relevant documentation
• I’ve added tests (if applicable)

🔗 Linked Issue

No issue created.

📖 Description

This PR introduces page-level feature flag protection via Nuxt middleware, enabling developers to guard entire pages with definePageMeta.

Key changes:

• ✨ Middleware:
  Added global route middleware (src/runtime/middleware/featureFlag.ts) that restricts access to routes based on feature flags.
  • Supports featureFlag for required feature
  • Optional featureFallback for redirects when disabled
  • Optional featureNotifyOnBlock for logging blocked access
• 📝 Documentation:
  Updated README.md with usage examples for definePageMeta to demonstrate page-level protection.
• 🏗️ Types: Introduced a new PageMeta interface (types/feature-flags.d.ts) to extend Nuxt schema with featureFlag, featureFallback, and featureNotifyOnBlock.
• 🧪 Tests:
  Added unit tests (test/unit/featureFlagMiddleware.test.ts) covering:
  • Redirect to fallback when feature is disabled
  • Error handling when disabled without fallback
  • Normal behavior when feature is enabled
• 🎮 Playground Example: Added a new demo page (playground/pages/new-protected.vue) showing feature-flag-based route protection in action.

This feature makes it easier to manage feature-gated access at the routing level, ensuring consistent user access control across the app.

🚨 Breaking Changes

• Yes
• No

🧪 Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🧹 Code style / formatting
• 🔨 Refactoring (no functional changes)
• 🧰 Tooling / CI
• 📝 Docs update
• 🧪 Tests
• 💡 Other (please describe):

🧩 Additional Context

Thank you for contributing! 💚