Implement Premium Entitlements and Feature Gating System

[Copilot]

This PR implements a comprehensive premium entitlements system for QuickNote Pro with Play Billing (Android) and StoreKit (iOS) integration, feature gating, and contextual upsells.

🚀 Key Features

Billing Integration

• Added in_app_purchase dependency for cross-platform billing
• Full Play Billing and StoreKit support with error handling
• Purchase restoration and verification
• Secure premium status persistence using SharedPreferences

Feature Gating System

The new FeatureGate utility provides centralized control over premium features:

// Voice notes: 10/month free → unlimited premium
final canRecord = FeatureGate.canRecordVoiceNote(currentCount, isPremium);

// Drawing: 1 layer free → 10 layers premium  
final canUseLayers = FeatureGate.canUseLayers(requestedLayers, isPremium);

// Export: TXT only → PDF/DOCX/MD formats
final canExportPDF = FeatureGate.canUseExportFormat('pdf', isPremium);

Non-Intrusive Upsells

Contextual upgrade prompts that appear when users try premium features:

// Pre-built upsell widgets
UpsellWidgets.voiceNotes(onDismiss: () => hideUpsell());

// Custom contextual upsells
ContextualUpsellWidget(
  featureName: 'advanced drawing',
  title: 'Professional Drawing Tools',
  subtitle: 'Unlock layers, effects, and advanced brushes',
  icon: Icons.brush,
);

Premium Service Architecture

The PremiumService extends ChangeNotifier for reactive state management:

• Real-time purchase status updates
• Automatic subscription expiry handling
• Graceful error handling for billing failures
• Development mode bypasses for testing

📱 Platform Configuration

Android

• Added com.android.vending.BILLING permission to AndroidManifest.xml
• Compatible with Google Play Console product configuration

iOS

• Ready for App Store Connect StoreKit configuration
• Supports both subscriptions and one-time purchases

🧪 Testing & Quality

• 32 comprehensive unit tests covering all feature gating scenarios
• Test coverage for billing flows, error handling, and edge cases
• Development mode toggles for easy testing
• Graceful degradation when billing services are unavailable

🎯 Premium Features Gated

Feature	Free Tier	Premium Tier
Voice Notes	10/month, 2min recordings	Unlimited, 1hr recordings, AI transcription
Drawing Tools	Basic tools, 1 layer	Advanced tools, 10 layers, effects
Export Formats	TXT only	PDF, DOCX, Markdown
Cloud Storage	Local only	10GB synced storage
Ads	Present	Ad-free experience

📚 Documentation

Added comprehensive documentation in docs/PREMIUM_SYSTEM.md with:

• Quick start guide and usage examples
• Implementation patterns and best practices
• Testing strategies and development workflows
• Platform-specific configuration details

🔄 Migration & Compatibility

• Existing premium upgrade UI enhanced with real billing
• Backward compatible with existing codebase
• API consistency with existing app patterns
• No breaking changes to current functionality

The implementation provides a complete foundation for monetizing QuickNote Pro while maintaining an excellent user experience with graceful feature degradation and non-intrusive upgrade prompts.

Fixes #25.

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