Feat/auto subtitle generation

[Bhavy12-cell]

Closes #793

What's included:

• A subtitle editor where users can add, edit and delete cues
• Each cue has a start time, end time and text field
• Auto-generate button that creates evenly spaced placeholder cues
• Export as SRT file so users can use subtitles in other tools
• Clear warning about increased export time when subtitles are used
• The auto-generate function is clearly marked in the code for
  future Whisper.js WASM integration

[vercel]

@Bhavy12-cell 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 Issues — @Bhavy12-cell

Please fix the following before your PR can be reviewed:

• ⚠️ Use a conventional PR title. Examples:
  • feat: add dark mode support
  • fix: resolve aria label missing on slider
  • docs: add deployment guide to README

Push new commits after fixing — this comment will update automatically.

📖 CONTRIBUTING.md

[github-actions]

👋 Thanks for your PR, @Bhavy12-cell!

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! 🎉