Add SEO/Open Graph metadata to homepage and add robots.txt

[joelnishanth]

Motivation

• Improve search engine indexing and social preview quality for the Apply landing page by adding canonical/robots metadata and social sharing tags.

Description

• Update index.html to change the page title and meta description and add a canonical URL and robots meta tag.
• Add Open Graph and Twitter meta tags including og:image and related image attributes for richer social previews.
• Add a new robots.txt that allows all user-agents and references https://apply.offlyn.ai/sitemap.xml.

Testing

• No automated tests were run for this change.

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Codex Task