fix(playback): manage Deno and retry YouTube sign-in failures

[wakamex]

I hit MEDIA_ERR_SRC_NOT_SUPPORTED and “Failed to load because no supported source was found” on Windows 11 with v0.5.1. Cached liked tracks played, but uncached search results and later radio tracks failed. yt-dlp rejected anonymous requests with a sign-in check, and the saved session also needed a JavaScript runtime to solve the player challenges.

This installs Deno 2.8.3 from its official release, verifies its SHA-256, and explicitly selects it for playback and metadata lookups. Playback starts anonymously and retries sign-in rejections once with a temporary copy of the selected account's saved cookies. The copy is removed after use; yt-dlp cannot overwrite the encrypted account jar. Deno is updated with app releases, while yt-dlp keeps its existing self-update behavior.

I pinned 2.8.3 to retain the original macOS 10.15 minimum on Intel and macOS 11 on Apple Silicon. Deno 2.9 introduced a macOS 12 requirement. The pin meets yt-dlp's minimum of Deno 2.3.0, but 2.8 is outside Deno's current LTS line.

• The updated Rust installer and downloader passed the original failing track on Windows with a fresh challenge cache. I observed Deno running both challenge lookups. The saved-session retry succeeded, the temporary cookie files were removed, and the complete 158.381-second Opus file decoded without errors.
• All 28 Rust tests pass locally, and the real Deno installation/reuse smoke test passes on Linux.
• The verified official macOS binaries declare deployment targets of 10.12 on Intel and 11.0 on Apple Silicon. Application testing on Catalina and Big Sur remains open.
• CI passed: frontend build, typecheck, lint and tests; Rust tests and real Deno installation/reuse checks on Windows, macOS and Linux.

Related: #83, #85.