Set up GitHub repo for discoverability

[kai-devxialabs]

Description

Optimize the public repo for GitHub discovery and SEO.

Tasks:

• Add relevant topics: nextjs, tailwindcss, dashboard, admin, template, react, typescript, open-source, free
• Add social preview image (1280x640 banner)
• Pin repo in DevxiaLabs org
• Add description and website URL
• Enable Discussions for community
• Add CONTRIBUTING.md
• Add issue templates (bug report, feature request)