docs: add Why Reframe comparison

[pranav28042007-boop]

Summary

• Add a factual "Why Reframe?" comparison section to the README
• Compare Reframe with CapCut, Kapwing, and Clipchamp
• Highlight privacy, local processing, account requirements, and open-source status

Closes #1777

Testing

README-only documentation change; no runtime tests required.

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[github-actions]

👋 Thanks for your PR, @pranav28042007-boop!

Welcome to Reframe — a browser-based video editor built for everyone 🎬

What happens next

1. 🤖 Automated checks — build & TypeScript typecheck will run automatically
2. ✅ Vercel preview — a preview deployment will be created (requires maintainer authorization for fork PRs)
3. 👀 Code review — a maintainer will review your changes
4. 🚀 Merge — once approved, your PR will be merged!

Quick checklist

• PR title follows Conventional Commits (e.g. feat: add dark mode)
• Linked the issue this PR closes (e.g. Closes #123)
• Tested the changes locally (bun run dev)
• Build passes (bun run build)

Useful links

• 📖 CONTRIBUTING.md
• 🐛 Open Issues
• ⭐ Star the repo to support the project!

Happy coding! 🎉

[github-actions]

✅ PR Format Check Passed — @pranav28042007-boop

Basic format checks passed. A maintainer will review your code changes.

This does not mean the PR is approved — it just means the format is correct.