feat: Implement Production-Ready Crypto Calculator with Enterprise Architecture

[cuambyte]

📝 Description

Complete implementation of cryptocurrency calculator challenge featuring real-time exchange rate conversion between crypto and fiat currencies.

✨ Features

• Real-time crypto/fiat currency conversion (USDT, VES, BRL, COP, PEN)
• Pixel-perfect UI with smooth animations
• Comprehensive input validation and error handling
• Currency swap functionality
• Exchange rate display (1 FIAT = X CRYPTO)

🏗️ Technical Implementation

• Architecture: Clean Architecture (Data/Domain/Presentation)
• State Management: Riverpod 3.0 with code generation
• Navigation: GoRouter 16.3.0 for type-safe routing
• HTTP Client: Dio 5.9 with error handling
• Testing: 11 unit tests with 100% pass rate
• Code Quality: Flutter 3.8+, SOLID principles, no deprecation warnings

🧪 Testing

• ✅ All tests passing (11/11)
• ✅ ValidateExchangeAmountUseCase tested
• ✅ CalculateCryptoExchangeUseCase tested
• ✅ Edge cases covered

📊 Code Quality

• ✅ flutter analyze - No issues found
• ✅ No deprecated APIs
• ✅ Centralized constants and strings
• ✅ Reusable widgets architecture

🎯 Architecture Highlights

• Use Cases pattern for business logic
• Repository pattern for data abstraction
• Immutable state with Freezed
• Error handling with Failure classes
• Separated concerns (UI/Logic/Data)