Implement Paywall and Upsell UX for Pro Themes and Monetization

[Copilot]

This PR implements a comprehensive paywall and monetization system for QuickNote Pro, enabling users to purchase premium themes and features through both monthly subscription ($2.99) and lifetime purchase ($14.99) options.

Key Features

Pro Theme System

• 3 Premium Themes: Futuristic (cyberpunk-inspired), Neon (vibrant creative), and Floral (nature-inspired)
• Visual Theme Picker: Interactive grid with live previews and Pro theme gating
• Seamless Integration: Accessible from Settings → Theme & Appearance with instant application

Paywall Experience

• Context-Aware Messaging: Different entry points (theme picker, settings) provide tailored upgrade flows
• Clear Value Proposition: Prominent "One-time purchase, no subscription" messaging for lifetime option
• Purchase Restoration: Reliable restore functionality for existing customers
• Error Handling: Graceful fallbacks when entitlement checks fail

Analytics & Business Intelligence

Comprehensive event tracking for optimization:

• paywall_shown - Track display frequency and entry points
• paywall_conversion - Monitor purchase success rates
• upsell_entry_point - Identify most effective upgrade triggers
• failed_payment - Debug payment issues and user friction

User Experience Flow

1. User selects Pro theme → Paywall displays with theme context
2. Clear pricing comparison with savings highlight (75% off for lifetime)
3. Purchase completion → Instant theme unlock and application
4. Pro status persists with visual indicators throughout app

Technical Implementation

Services Architecture

• ThemeEntitlementService: Manages Pro access, purchase state, and subscription validation
• PaywallAnalyticsService: Tracks business metrics with structured event logging
• Enhanced ThemeService: Dynamic theme loading with Pro theme support

Quality Assurance

• Comprehensive Test Coverage: Unit tests for entitlement logic, purchase flows, and analytics
• Developer Tools: Debug-only testing utilities for rapid development iteration
• Error Resilience: Graceful degradation when services are unavailable

Accessibility & Localization

• Global Ready: Structured for multi-language and regional pricing support
• Screen Reader Compatible: Proper semantic markup and focus management
• High Contrast Support: Pro themes maintain accessibility standards

Demo Usage

// Check theme access
if (entitlementService.shouldShowPaywallForTheme('futuristic')) {
  // Show paywall with context
  PaywallAnalyticsService.logPaywallShown(
    entryPoint: 'theme_picker',
    featureType: 'theme',
    specificFeature: 'futuristic',
  );
}

// Grant access after purchase
await entitlementService.grantPremiumAccess(purchaseType: 'lifetime');
await themeService.setSelectedTheme('futuristic');

The implementation provides a foundation for sustainable monetization while maintaining excellent user experience. All paywall interactions are non-intrusive, clearly communicated, and instantly reward successful purchases.

Fixes #37.

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