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.