Add download options with embed chapters support

[elewashy]

Summary

Adds global download options with initial support for embedding chapters.

Motivation

Videos with timestamps can preserve those chapters in downloaded files, while keeping the default paste → format → download flow unchanged.

Closes #3.

Architecture

• Introduces an immutable DownloadOptions model for global download preferences.
• CLI parsing and the TUI both produce the same complete options model.
• The download layer receives that model and is the only layer that translates enabled options into yt-dlp arguments.

UX

• Adds --embed-chapters for CLI use.
• From the format picker, ^o opens a lightweight Download Options dialog.
• Space toggles the highlighted option, Enter applies the draft, and Esc discards it.
• Download formats remain dedicated to formats; the dialog's bounded selector can grow with future global options.

Testing

• npm run typecheck
• npm test
• npm run build