feat: add playlist, recent files and MPRIS backends

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

autotests/CMakeLists.txt

@@ -7,3 +7,9 @@ ecm_add_test(mpvuserconfigtest.cpp
     TEST_NAME mpvuserconfigtest
     LINK_LIBRARIES qtpotato_core Qt6::Test
 )
+
+ecm_add_test(playlistmodeltest.cpp ../src/playlist/playlistmodel.cpp
+    TEST_NAME playlistmodeltest
+    LINK_LIBRARIES Qt6::Qml Qt6::Test
+)
+target_include_directories(playlistmodeltest PRIVATE ../src/playlist)