Add linked URL playlist import, refresh, and slugified filenames

[pythrick]

Summary

• add persistent URL import mode (U) using Playlist Name | URL
• store playlist link metadata as top-level source_url in local playlist TOML files
• add playlist-manager refresh action (R) to re-sync linked playlists from source_url
• preserve source_url on normal local playlist edits
• slugify playlist filenames for new playlists while keeping backward compatibility with existing non-slug filenames
• add tests for URL import parsing, linked playlist metadata persistence, and slug/legacy path behavior

Validation

• go test ./... -count=1

Closes #100

[pythrick]

Superseded by #108 (dedicated feature branch ). Continuing updates there.

[pythrick]

Closing in favor of #108 to avoid tracking changes from fork .

[pythrick]

Superseded by #108 (dedicated feature branch feat/linked-url-playlists-pr101). Continuing updates there.