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
Supersedes #101

[pythrick]

This PR replaces #101 and carries the same scope on a dedicated feature branch (not fork main).

[bjarneo]

I need to test this PR myself because the keybindings are getting messy. Especially since r is something else completely.