Enhancement: Linked local playlists from URL with refresh support

[pythrick]

Summary

Add first-class support for creating persistent local playlists from remote URLs, with a stored source link and manual refresh.

Problem

Right now URL-based playback/import can be transient. Users want to:

• persist a playlist created from URL
• keep it tied to its remote source
• refresh it later without recreating it manually

Proposed Solution

1. Add URL import mode for persistent playlists:

   • Input format: Playlist Name | URL
   • Resolve URL using existing remote resolver
   • Save tracks as local TOML playlist

2. Store playlist source metadata:

   • Add top-level source_url in local playlist TOML

3. Add manual sync action:

   • In playlist manager, add R to refresh selected linked playlist from source_url
   • Overwrite track list with latest remote contents
   • Keep source_url preserved

4. Keep local UX predictable:

   • Show success/failure status messages for import/refresh
   • If source_url missing, show a clear “not linked” error on refresh

Nice-to-Have Follow-ups

• Allow URL-only import with auto-generated playlist name from remote metadata
• Optional startup auto-refresh for linked playlists (sync_linked_playlists = true)
• Per-playlist conflict mode (overwrite vs merge/dedup)

Acceptance Criteria

• User can import Name | URL into persistent local playlist
• Saved playlist contains source_url
• Pressing R refreshes linked playlist successfully
• Existing non-linked playlists remain fully functional
• Existing playlists keep working (backward compatible file parsing)