fix(frontend): Implement localStorage persistence for chat session history (#45)

[SandeepChauhan00]

🔗 Related Issue

Closes #45

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📋 Summary

This PR implements persistent chat session history using localStorage, ensuring users don't lose their conversation when refreshing the page or accidentally closing the browser.

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🐛 Problem

The current implementation intentionally clears all stored chat history on every page load, causing users to lose their conversation on refresh.

Impact:

• Users lose entire conversation on page refresh (F5)
• Research progress is lost unexpectedly
• Poor user experience during long research sessions

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✅ Solution

Feature	Implementation
Load History	getInitialMessages() restores saved conversations
Save History	useEffect hook saves messages on state change
Date Handling	Converts timestamp strings back to Date objects
Clear Function	Updated to also clear localStorage

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🔄 Changes Made

• Removed code that clears localStorage on component mount
• Added getInitialMessages() helper function
• Added WELCOME_MESSAGE constant
• Added useEffect hook to persist messages
• Updated clearChat() function

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🧪 Testing Performed

Test Case	Result
Page refresh (F5) - messages persist	✅ Pass
Browser close/reopen - messages persist	✅ Pass
Clear button functionality	✅ Pass
Welcome message for new users	✅ Pass

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✅ Checklist

• Code compiles without errors
• Tested locally
• Related issue linked ([Bug] Chat conversation history is lost on page refresh - No session persistence #45)
• Ready for review