feat: display friendly error messages on API failure

[Ai-chan-0411]

Summary

Closes #34

Adds user-friendly error messages when the GitHub API fails, replacing generic error text with specific, actionable messages.

Changes

API route (app/api/compare/route.ts)

• Added classifyError() helper that maps different failure modes to clear messages:
  • User not found (404): "GitHub user not found. Please check the username and try again."
  • Rate limit exceeded (429): "GitHub API rate limit exceeded. Please wait a few minutes and try again."
  • Auth error (401/500): "GitHub API authentication error. Please contact the administrator."
  • Network error (503): "Unable to reach GitHub. Please check your connection and try again."
  • Generic fallback: "Something went wrong while fetching data. Please try again later."
• Returns appropriate HTTP status codes for each error type

Page (app/page.tsx)

• Passes error prop to CompareForm component for inline error display
• Removed duplicate error display below the form (the CompareForm already has a styled Alert for errors)

Before / After

Before	After
"Failed to calculate score"	"GitHub user not found. Please check the username and try again."
"Failed to calculate score"	"GitHub API rate limit exceeded. Please wait a few minutes and try again."

[vercel]

@aoi-dev-0411 is attempting to deploy a commit to the osama's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

Thank you for the pull request! ✅

A maintainer will review this soon. Please be patient while we take a look. 🙌

[Ai-chan-0411]

Closing this PR as there has been no maintainer feedback for several weeks. Thank you for the opportunity to contribute! Feel free to reopen if you would like these changes.