fix: sanitize and validate GitHub username input

[shagunkillekar5]

Description

Implemented GitHub username sanitization and validation in the landing page input field.

Changes made

• Trimmed whitespace from usernames
• Extracted username automatically from GitHub profile URLs
• Added validation for invalid GitHub username characters
• Prevented invalid submissions
• Added user-friendly error message for invalid usernames

Tested

• Valid usernames work correctly
• Invalid usernames show error
• GitHub URLs are sanitized properly

Fixes #63

[vercel]

@shagunkillekar5 is attempting to deploy a commit to the jhasourav07's projects Team on Vercel.

A member of the Team first needs to authorize it.

[JhaSourav07]

CI is failing can you fix it and also PR description should be as template

[github-actions]

👋 Hey @shagunkillekar5, it looks like you didn't use our PR template!

The section ## Pillar is missing from your PR description.

Please update your PR description to include all required sections so we can review this properly:

• ## Description — What does this PR do? Which issue does it fix?
• ## Pillar — Which contribution pillar does this fall under?
• ## Checklist — Have you ticked off the quality checklist?

You can find the full template in CONTRIBUTING.md. Just edit your PR description and the needs-details label will be removed automatically. 🙌

[github-actions]

⚠️ Hey @shagunkillekar5, this PR has merge conflicts with the main branch.

Please pull the latest changes and resolve the conflicts so we can review it!

git fetch origin
git rebase origin/main
# resolve any conflicts, then:
git push --force-with-lease

Once resolved, the needs-rebase label will be removed automatically on the next check. 🙌