Implement Premium Entitlements and Feature Gating with Cross-Platform Billing

[Copilot]

This PR implements a comprehensive premium entitlement system that gates advanced features behind Play Billing (Android) and StoreKit (iOS) purchases, providing a seamless upgrade experience for users.

🎯 Key Features Implemented

Premium Feature Gating:

• Voice note transcription with usage limits (10/month for free users, unlimited for premium)
• Recording time limits (60 seconds for free, unlimited for premium)
• Advanced drawing tools and color palettes restricted to premium users
• Export formats (PDF/Word premium, TXT free) with contextual upgrade prompts
• Foundation for cloud sync, advanced search, and custom themes

Cross-Platform Billing Integration:

• BillingService handles Play Billing and StoreKit with automatic product loading
• Real-time pricing display from app stores ($1.00/month, $5.00 lifetime)
• Purchase verification and restoration with proper error handling
• Offline entitlement caching for reliable feature access

Smart User Experience:

• PremiumGateWidget provides two modes: full upsell view and read-only overlay
• Contextual upgrade dialogs with feature-specific benefits and messaging
• Non-intrusive premium indicators using consistent amber branding
• Graceful limit enforcement with clear upgrade paths

🔧 Technical Implementation

Architecture:

• EntitlementService manages premium status with local SharedPreferences caching
• Integration with existing provider-based state management
• Feature limits enforced in real-time with usage tracking
• Developer override functionality for testing premium features

UI Integration:

• Updated voice input widget to check transcription entitlements and recording limits
• Enhanced drawing canvas with premium tool restrictions and upgrade prompts
• Modified export functionality to gate PDF/Word behind premium subscription
• Connected premium upgrade screen to actual billing service for purchases

Quality Assurance:

• Comprehensive unit tests for billing and entitlement services
• Integration tests covering premium feature flows
• Example screen demonstrating all premium gate patterns
• Mock services for reliable testing without store dependencies

🎨 User Journey

Free users see beautiful previews of premium features with clear value propositions. When they hit limits (like 10 voice transcriptions per month), contextual dialogs explain the benefits of upgrading. The purchase flow uses real store pricing and handles edge cases gracefully.

Premium users get instant access to all features with subtle premium badges, creating a premium feel without being intrusive.

📱 Platform Support

• Android: Billing permission added, ready for Google Play Console product configuration
• iOS: StoreKit integration ready for App Store Connect setup
• Web: Graceful fallback handling for unsupported billing operations

The implementation follows existing app patterns and maintains backward compatibility while providing a solid foundation for future premium features.

Fixes #29.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.