Add is_dynamic field to PlayerQueue

[dmoo500]

Summary

Adds an is_dynamic field to PlayerQueue so all clients can check directly whether a dynamic playlist is the active radio source, without having to inspect radio_source manually.

Background

Previously, clients had to implement the detection logic themselves:

len(queue.radio_source) == 1
and isinstance(queue.radio_source[0], Playlist)
and queue.radio_source[0].is_dynamic

This was duplicated in the web frontend and would need to be reimplemented in every client (mobile app, etc.).

Change

Adds is_dynamic: bool = False as a serialized field on PlayerQueue. The server sets this field whenever radio_source changes.

Why a stored field instead of @property: PlayerQueue uses mashumaro's DataClassDictMixin. Its to_dict() / to_cache() serialization only processes dataclass fields — @property accessors are silently skipped and would never reach the wire or the cache, so clients would always see the default value. Verified with a minimal test against the installed mashumaro version.

Related

• Server PR: Set PlayerQueue.is_dynamic when radio_source changes server#3886
• Frontend PR: Use PlayerQueue.is_dynamic directly frontend#1773
• Mobile app issue: Hide radio mode button when playing a dynamic playlist mobile-app#370