enhancement: Sanitize developer usernames by stripping leading '@' and GitHub URLs #254

When users paste or type usernames with a leading @ (e.g. @torvalds) or paste a full GitHub profile URL (e.g. https://github.com/octocat), the application passes the raw string to GitHub GraphQL. The GraphQL API returns a 404 / User Not Found error.

Motivation

Users frequently copy developer handles with @ or paste profile URLs directly from their browser address bar. Automatically sanitizing inputs prevents unnecessary API errors and simplifies user flow.

Tasks

• Create a helper cleanGitHubUsername(input: string): string in src/utils/github.ts (or src/utils/string.ts).
• Handle leading @ removal (e.g., @shadcn -> shadcn).
• Extract username from GitHub URLs (e.g., https://github.com/shadcn/ -> shadcn).
• Trim whitespace and trailing slashes.
• Use cleanGitHubUsername in CompareForm inputs, src/app/api/compare/route.ts, and src/app/api/user/[username]/route.ts.
• Add unit tests covering various input formats.

Files to Check

• src/utils/github.ts
• src/features/comparison/components/compare-form.tsx
• src/app/api/compare/route.ts
• src/app/api/user/[username]/route.ts