feat(auth): implement OTP-based authentication MVP

[Aqsa30nz]

Summary

Implements the MVP OTP-based authentication flow and integrates the frontend /auth page with backend authentication endpoints, providing a complete end-to-end login experience using phone number verification.

Changes

Backend

• Added POST /auth/otp/request
• Added POST /auth/otp/verify
• Generate 6-digit OTPs for authentication
• Store OTPs with expiration timestamps
• Validate the latest unused OTP for a phone number
• Mark OTPs as used after successful verification
• Automatically create a user on first successful login
• Generate JWT access tokens after authentication
• Return authenticated user details in the response

Frontend

• Implemented OTP authentication page at /auth
• Added phone number submission flow
• Added OTP verification flow
• Integrated frontend with backend authentication endpoints
• Store JWT token in localStorage after successful login
• Added authentication status indicator in the navbar
• Added logout functionality
• Added error handling for invalid OTPs and failed requests

Tested

Success Cases

• OTP request endpoint returns a valid response
• OTP verification successfully authenticates users
• JWT token is returned after successful verification
• New users are automatically created on first login
• User login state persists via localStorage
• Logout removes stored authentication data and updates the UI

Failure Cases

• Invalid OTP
• Expired OTP
• Missing phone number
• Missing OTP
• Backend request failures

Impact

• Provides a complete end-to-end OTP authentication flow
• Enables seamless frontend and backend authentication integration
• Establishes JWT-based session handling for authenticated users
• Removes dependency on password-based authentication for MVP scope
• Creates a foundation for future protected features such as wishlists, bookings, and user-specific actions

Fixes #51

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