Implement comprehensive pricing tiers and free limits system with analytics

[Copilot]

This PR implements a complete pricing tiers and free limits system for Quicknote Pro, providing clear upgrade paths and comprehensive monetization analytics.

Key Features Implemented

Pricing Tier System

• Free Tier: 100 notes max, 10 voice notes/month, 5 exports/month, 3 attachments per note (5MB each), local storage only
• Premium Tier: Unlimited everything, cloud sync, advanced drawing tools, custom themes, ad-free experience, OCR text recognition

Smart Limit Enforcement

The new LimitEnforcementService provides contextual limit checking throughout the app:

// Check if user can create a note
final result = await limitService.canCreateNote(currentNoteCount);
if (!result.allowed) {
  // Show upgrade prompt with specific messaging
  LimitReachedPrompt.showNoteLimit(context);
}

Subscription Management

The PricingTierService handles all subscription states including trials, monthly subscriptions, and lifetime purchases:

// Start free trial
await pricingService.startFreeTrial(trialDays: 7);

// Activate premium subscription
await pricingService.activatePremiumSubscription(
  subscriptionType: SubscriptionType.lifetime,
  productId: ProductIds.premiumLifetime,
);

Monetization Analytics

Comprehensive event tracking for conversion funnel analysis:

• freeLimitReached - When users hit any limitation
• upgradeInitiated - When upgrade process starts
• upgradeCompleted - Successful purchases
• trialStarted - Free trial activations

User Experience Enhancements

• Context-aware upgrade prompts: Different messaging based on which limit was reached
• Usage status widgets: Show current usage and remaining limits
• Trial management: 7-day free trials with clear expiration warnings
• Graceful degradation: Premium features are gated but don't break free user experience

Updated Pricing Structure

• Monthly: $2.99/month (was $1.00)
• Lifetime: $14.99 one-time (was $5.00)
• Free Trial: 7 days with full premium access
• Value proposition: Lifetime saves 75% vs monthly

Technical Implementation

The system is built with a clean service-oriented architecture:

• Models define pricing tiers and user entitlements
• Services handle business logic and persistence
• UI components provide consistent upgrade messaging
• Comprehensive unit tests ensure reliability

All changes maintain backward compatibility and include proper error handling for edge cases like network failures, subscription renewals, and platform store integration issues.

Fixes #43.

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