refactor: split PlayQueue from header-only into .hpp/.cpp pair

Move all method implementations from playqueue.hpp into a new
playqueue.cpp, keeping only declarations and trivial inline getters
in the header. No logic or behavior changes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/CMakeLists.txt

@@ -6,8 +6,9 @@ target_sources(qobuz-qt PRIVATE
     mainwindow.hpp
     mainwindow.cpp
 
-    # Queue (header-only)
+    # Queue
     playqueue.hpp
+    playqueue.cpp
 
     # Backend (Qt wrapper around Rust FFI)
     backend/qobuzbackend.hpp