feat(ui): add EnableNowPlaying configuration (default true) (#4219)

* Add EnableNowPlaying config option

* Return 501 for disabled NowPlaying

* chore(tests): remove get_now_playing_route test

* Disable now playing events when disabled

* fix(tests): add mutex for thread-safe access to scrobble buffer

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

tests/mock_data_store.go

@@ -2,6 +2,7 @@ package tests
 
 import (
 	"context"
+	"sync"
 
 	"github.com/navidrome/navidrome/model"
 )
@@ -25,6 +26,7 @@ type MockDataStore struct {
 	MockedUserProps      model.UserPropsRepository
 	MockedScrobbleBuffer model.ScrobbleBufferRepository
 	MockedRadio          model.RadioRepository
+	scrobbleBufferMu     sync.Mutex
 }
 
 func (db *MockDataStore) Library(ctx context.Context) model.LibraryRepository {
@@ -193,6 +195,8 @@ func (db *MockDataStore) Player(ctx context.Context) model.PlayerRepository {
 }
 
 func (db *MockDataStore) ScrobbleBuffer(ctx context.Context) model.ScrobbleBufferRepository {
+	db.scrobbleBufferMu.Lock()
+	defer db.scrobbleBufferMu.Unlock()
 	if db.MockedScrobbleBuffer == nil {
 		if db.RealDS != nil {
 			db.MockedScrobbleBuffer = db.RealDS.ScrobbleBuffer(ctx)