Add complete authentication system with protected routes

[jikrana1]

📌 Related Issue

Fixes: #7

🔍 Describe your changes?

• Added user registration functionality
• Added user login with JWT authentication
• Added logout functionality
• Implemented protected routes for authenticated users
• Added authentication state handling using localStorage
• Updated navbar with conditional rendering based on authentication status
• Displayed Create Product button only for logged-in users
• Added premium UI for Login, Signup, and Navbar components
• Improved user experience with redirects and authentication checks

📸 Screenshot

🧪 Checklist

• I have tested my changes locally
• Authentication works correctly
• Protected routes are functioning
• No existing functionality was broken
• Code follows project structure and standards

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[rishima17]

@jikrana1 Please resolve the conflicts.

[Aamod007]

Hi there! Implementing a complete authentication system is a massive and important feature. Unfortunately, there are merge conflicts right now. Please rebase or pull the latest changes from main to resolve them, and I'll be happy to take a deeper look!

[Aamod007]

Hi! Recent merges to the main branch have caused some merge conflicts here. Could you please pull the latest changes from main, resolve the conflicts, and update this PR? Thank you!