Modernize Deno and GitHub Actions maintenance

[AnandChowdhary]

Summary

• Add a Deno config with reusable update and check tasks plus a lockfile
• Move the fetcher to Deno 2-compatible, pinned imports and fix existing type-check errors
• Add CI for formatter/lint/type checks on PRs and pushes to main
• Update Actions to Node 24-compatible versions and migrate Pages deployment to official GitHub Pages actions
• Add Dependabot coverage for GitHub Actions

Verification

• deno task check
• deno task update completed locally with Meta has not changed