fix(frontend): sanitize API error messages to prevent info disclosure

[SandeepChauhan00]

🔗 Related Issue

Closes #47

📋 Summary

Sanitizes API error messages to prevent exposing internal system details to users.

🐛 Problem

When API errors occur, raw error responses including sensitive details are displayed:

• LLM model names (gemini-2.5-flash-lite)
• Pricing tiers (FreeTier)
• Rate limit thresholds (quotaValue: 20)
• SSL/Protocol errors (_ssl.c:1032)

This violates OWASP security best practices.

✅ Solution

Added ErrorHandler.ts utility that:

• Sanitizes all error responses before displaying to users
• Shows user-friendly messages based on error type
• Logs full error details to console for debugging

📁 Files Changed

File	Change
src/utils/ErrorHandler.ts	NEW - Error sanitization utility
src/App.tsx	Updated to use sanitized error messages

🔄 Before & After

Error Type	Before (Exposed)	After (Sanitized)
Rate Limit	429 RESOURCE_EXHAUSTED gemini-2.5-flash-lite	"I'm currently experiencing high demand..."
SSL Error	[SSL: UNEXPECTED_EOF] _ssl.c:1032	"Unable to connect to the server..."
Timeout	504 Gateway Timeout	"The request took too long..."

🧪 Testing Performed

Test Case	Result
Rate limit error sanitized	✅ Pass
SSL error sanitized	✅ Pass
Network error sanitized	✅ Pass
Full error logged to console	✅ Pass
Normal responses unaffected	✅ Pass

✅ Checklist

• Code compiles without errors
• Tested locally
• Related issue linked ([Security/UX] API Error Messages Expose Internal System Details to Users #47)
• Ready for review

[SandeepChauhan00]

Hi team! 👋

Following up on this security enhancement PR. This addresses Issue #47
by preventing internal system details from being exposed to end users
in error messages.

Changes:

• ✅ Generic error messages for client-side display
• ✅ Detailed errors still logged server-side for debugging
• ✅ Improved security posture without losing observability

This is a low-risk change that improves security. I've completed all
the checklist items and the implementation is ready for review.

Would appreciate your feedback when you have a moment. Happy to discuss
the approach or make adjustments!

Thanks!