Implement complete initial project structure for Enterprise Expense Tracker

[Copilot]

This PR creates the complete initial project structure for the Enterprise Expense Tracker application as specified in the requirements. The implementation provides a solid foundation with modern React, TypeScript, MobX state management, and Tailwind CSS styling.

What's Implemented

Core Application Structure

• React 18 with TypeScript setup and proper entry points (src/index.tsx, src/App.tsx)
• React Router DOM v6 for navigation with initial routing structure
• Tailwind CSS configuration with custom color palette and responsive design
• Production-ready build system with optimized output

State Management with MobX

• RootStore as the central store coordinator
• AuthStore with complete authentication flow including login, logout, and user management
• ExpenseStore with full CRUD operations, expense filtering, and computed values for analytics
• Reactive state updates with proper TypeScript integration

Component Architecture

• Organized component hierarchy with dedicated folders for different domains:
  • layout/ - Header, Footer, and Layout wrapper components
  • expenses/ - ExpenseList and ExpenseForm components for expense management
  • auth/ - LoginForm component for authentication
  • common/ - Reusable Button and LoadingSpinner components
• Each component folder includes index files for clean imports

API Services Layer

• api.ts - Axios-based HTTP client with request/response interceptors for authentication and error handling
• authService.ts - Complete authentication API including login, registration, password reset, and token management
• expenseService.ts - Comprehensive expense management API with support for CRUD operations, file uploads, approvals, and statistics

Utility Functions and Types

• types.ts - Complete TypeScript interfaces for User, Expense, ExpenseCategory, and API responses
• formatters.ts - Utility functions for currency formatting, date handling, number formatting, and text manipulation
• validators.ts - Form validation functions for email, password strength, expense amounts, file uploads, and more

Key Features

TypeScript Best Practices

• Comprehensive type definitions for all data structures
• Proper interface segregation and generic type usage
• Strict TypeScript configuration with full type safety

Modern React Patterns

• Functional components with hooks
• Observer pattern integration with MobX for reactive updates
• Proper prop typing and component composition

Responsive Design

• Tailwind CSS with custom configuration
• Mobile-first responsive design approach
• Consistent spacing and color scheme

Error Handling

• HTTP interceptors for global error handling
• Proper error states in stores
• Validation with user-friendly error messages

Development Experience

• Hot module replacement for development
• Production-optimized builds
• Proper .gitignore for Node.js projects

Testing

The application successfully:

• ✅ Compiles with TypeScript without errors
• ✅ Builds for production (70.69 kB gzipped main bundle)
• ✅ Starts development server and renders correctly
• ✅ Includes proper routing structure with navigation

The foundation is now ready for implementing specific features like expense forms, user authentication flows, expense approval workflows, and dashboard analytics.

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