Fix YouTube previews and downloads with yt-dlp, Deno, and optional Firefox cookies

[DrRedBird]

Summary

Fixes #25.

This change addresses YouTube preview and download failures caused by authentication and JavaScript challenge requirements.

Changes

• Installs yt-dlp[default] so yt-dlp-ejs and the recommended dependencies are available.
• Detects an existing supported Deno runtime.
• Installs a checksum-verified, ThemeDeck-managed Deno runtime when needed.
• Passes Deno's explicit executable path to yt-dlp using --js-runtimes.
• Adds an optional Use Firefox cookies for YouTube setting, disabled by default.
• Saves the Firefox-cookie setting between ThemeDeck restarts.
• Keeps using the verified local yt-dlp fallback when the virtual-environment update fails.
• Shows yt-dlp and Deno readiness in the ThemeDeck interface.
• Adds automated tests for yt-dlp installation, Deno setup, Firefox-cookie configuration, and fallback selection.
• Documents the YouTube support setup.