Implement comprehensive analytics and event taxonomy for monetization and usage tracking

[Copilot]

This PR implements a complete analytics system for tracking monetization and feature usage in Quicknote Pro, providing structured event taxonomy, privacy-compliant data collection, and business intelligence capabilities.

Overview

The analytics system introduces a comprehensive event tracking framework that captures user interactions across all key app functions while maintaining strict privacy controls and GDPR compliance. The implementation focuses on monetization insights, feature usage patterns, and user engagement metrics.

Key Components

Analytics Event System

• AnalyticsEvent: Privacy-safe event model with automatic PII filtering and JSON serialization
• AnalyticsEventType: Comprehensive taxonomy of 50+ categorized events covering monetization, feature usage, themes, errors, and engagement
• Event Properties: Structured schema including entry_point, method, value, conversion, and error_code for rich context

Core Analytics Service

• AnalyticsService: Central tracking service with user consent management, offline event queuing, and batch processing
• Privacy Controls: Explicit user opt-in, data clearing capabilities, and privacy-safe event processing
• Error Handling: Comprehensive fallback mechanisms, retry logic, and graceful degradation

Integration Layer

• AnalyticsIntegrationService: Seamless integration with existing services (theme, notes, sync) for automatic event tracking
• Specialized Methods: Dedicated tracking for monetization flows, usage patterns, and error scenarios
• Theme Integration: Automatic tracking of theme changes and customization preferences

Event Categories

The system tracks events across seven major categories:

1. Monetization (12 events): Premium purchases, subscriptions, paywall interactions, free limit encounters
2. Advertising (7 events): Ad lifecycle, user interactions, rewarded ad completion
3. Theme (4 events): Theme mode changes, accent color selection, premium theme access
4. Feature Usage (12 events): Note operations, voice recording, OCR, cloud sync, backup/import
5. Navigation/UX (7 events): App lifecycle, onboarding, settings access, help usage
6. Error/Performance (5 events): Application errors, network issues, crashes, performance problems
7. Engagement (6 events): Session tracking, retention metrics, feature discovery

Privacy and Compliance

The implementation prioritizes user privacy with:

• Explicit consent required before any tracking begins
• Automatic PII filtering and data anonymization
• Complete data clearing capabilities for GDPR compliance
• Local event processing with privacy-safe transmission
• User control over analytics preferences

Business Intelligence Ready

Comprehensive documentation provides:

• Dashboard requirements for revenue, engagement, and performance KPIs
• Real-time monitoring specifications for critical alerts
• Integration examples for all major app components
• Reporting automation and visualization requirements

Testing Coverage

Extensive test suite includes:

• Unit tests for event models, serialization, and privacy filtering
• Service tests for consent management, event tracking, and error handling
• Integration tests demonstrating real-world usage scenarios
• Privacy compliance validation and data handling verification

Example Usage

// Initialize analytics with privacy controls
final analyticsService = AnalyticsService();
await analyticsService.initialize();
await analyticsService.setUserConsent(true);

// Track monetization events
await integrationService.trackPremiumPurchaseStarted(
  entryPoint: AnalyticsEntryPoint.paywall,
  productId: 'premium_monthly',
  price: 1.0,
);

// Track feature usage automatically
await integrationService.trackNoteCreated(
  entryPoint: AnalyticsEntryPoint.dashboard,
  hasAttachments: true,
  attachmentCount: 2,
);

The analytics system provides the foundation for data-driven decision making while maintaining the highest standards of user privacy and technical reliability.

Fixes #39.

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