feat: add custom 404 error page with branding and responsive design

[AbhishekChamp879]

Description

Removes src/app/not-found.tsx which is incompatible with Next.js static export (output: "export" in next.config.ts). This file breaks next build in production.

Replaced with a standalone public/404.html static page that:

• Preserves the original 404 design (Reframe branding, "Frame not found" copy, back link)
• Supports dark mode via prefers-color-scheme using the project's CSS design tokens
• Works on all static hosting providers (Vercel, Netlify, GitHub Pages)
• Requires zero JavaScript or React — pure HTML/CSS

This aligns with the critical rule documented in CLAUDE.md :
not-found.tsx is incompatible — reject PRs adding it

Related Issue

Fix #697

Type of Contribution

• Bug fix
• New feature
• Documentation update
• GSSoC contribution

Participant Info

• GitHub username: https://github.com/AbhishekChamp879
• Contribution level (Beginner/Intermediate/Advanced): Intermediate

Checklist

• I have read the contribution guidelines
• My changes follow the project structure
• I have tested my changes in Chrome, Firefox, and Safari
• bun run lint passes (no ESLint errors)
• bunx tsc --noEmit passes (no TypeScript errors)
• New interactive elements have aria-label / accessible names
• No console.log statements left in
• This PR is related to a valid issue
• Screen recording attached above (required for UI/feature/design changes)

[vercel]

@AbhishekChamp879 is attempting to deploy a commit to the magic-peach1's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

✅ PR Format Check Passed — @AbhishekChamp879

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.

[github-actions]

👋 Thanks for your PR, @AbhishekChamp879!

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

🟠 GSSoC'26 PR detected — thanks for contributing under GirlScript Summer of Code 2026!

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! 🎉