Show warning when non-existent usernames are entered

Update CLAUDE.md with deployment URL, request flow, and key constraints

Add two custom overlay mode examples to README

Use gvanrossum and apoorvdarshan for README example graphs

Use torvalds and apoorvdarshan for README example graphs

Replace torvalds with sindresorhus in README examples for better visual contrast

Use torvalds and gvanrossum for all README example graphs

Update URLs to github-contribution-merger.vercel.app and replace example users with own graphs

Add Vercel Web Analytics script

Trigger Vercel deployment

Add Code of Conduct (Contributor Covenant v2.1)

Add email to security policy

Add SECURITY.md with vulnerability reporting policy

Add CONTRIBUTING.md with setup and guidelines

Update GitHub Sponsors username in FUNDING.yml