Implement comprehensive ads integration system for free tier with smart timing and A/B testing

[Copilot]

This PR implements a complete ads system for QuickNote Pro's free tier, providing monetization while maintaining excellent user experience through intelligent ad placement, frequency capping, and A/B testing capabilities.

Key Features

🎯 Smart Ad System

• AdsService: Central service managing all ad operations with Provider integration
• Smart Interstitial Timing: Intelligent display logic with 15-25% probability based on user behavior
• Frequency Capping: Time-based and session-based limits to prevent user fatigue
• A/B Testing Framework: Automatic variant assignment for optimization

📱 Ad Formats & Placements

• Banner Ads: 4 sizes (standard, large, medium, leaderboard) with responsive design
• Native Ads: 3 templates that blend seamlessly with app content
• Interstitial Ads: Full-screen overlays with smooth animations and countdown timers

Integrated across all major app screens:

• Notes list: Banner ads every 5 notes + interstitials on note opening
• Search results: Native ads in suggestions + banner ads every 4 results
• Settings: Medium native ad below premium section
• Folder organization: Native ads in grid layout
• Navigation transitions: Smart interstitials on important actions

📊 Analytics & Insights

• Comprehensive Tracking: impression, click, dismiss, conversion, blocked, failure events
• Real-time Metrics: CTR, conversion rate, eCPM, revenue calculations
• User Behavior Analytics: Session tracking with persistent storage
• A/B Testing Data: Variant performance tracking for optimization

🛡️ User Experience & Privacy

• Premium Integration: Automatic ad disabling for premium users with conversion tracking
• Graceful Fallbacks: Promotional content when ads fail to load
• Clear Labeling: All ads marked as advertisements with close buttons
• Privacy Compliant: Anonymous analytics, local storage, GDPR/CCPA ready

🔧 Developer Experience

• Easy Integration: Simple widgets (SimpleBannerAd, SimpleNativeAd)
• Configuration-Driven: Centralized settings in AdsConfig
• Comprehensive Testing: Unit tests covering all components
• Complete Documentation: Implementation guide with examples

Implementation Highlights

The system uses intelligent probability-based display logic:

// Smart interstitial with 25% chance on important transitions
await SmartInterstitialHelper.showSmartInterstitial(
  context,
  AdsConfig.placementNoteList,
  isImportantTransition: true,
);

// Simple native ad integration
const SimpleNativeAd(
  placementId: AdsConfig.placementSettings,
  template: NativeAdTemplate.medium,
)

Frequency caps prevent user fatigue:

• Banner ads: No cap (non-intrusive)
• Interstitial ads: 30 minutes between displays
• Native ads: 5 minutes between displays
• Session limits per placement (5-15 ads)

The analytics system tracks comprehensive metrics:

• Revenue attribution to specific ad placements
• User interaction patterns for optimization
• A/B testing performance data
• Premium conversion tracking

Architecture

The implementation follows a clean, extensible architecture:

• Models: AdPlacement, AdInstance, AdAnalytics for type safety
• Services: AdsService with Provider integration for state management
• Widgets: Reusable ad components with consistent theming
• Configuration: Centralized settings for easy customization

All ads are designed to be easily replaceable with real ad network SDKs while maintaining the same interface and user experience.

Fixes #41.

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