gate scrobbling behind a setting and funnel server writes through a sink

Scrobbling can be turned off in Settings (on by default). Now-playing,
scrobbles and stars all go through a ServerActionSink so a persistent
offline outbox can replace the network implementation later without
touching the callers.

app/src/main/kotlin/com/subtracks/ui/settings/SettingsViewModel.kt

@@ -48,6 +48,11 @@ class SettingsViewModel(
             .syncConcurrency()
             .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), DEFAULT_FETCH_CONCURRENCY)
 
+    val scrobbling: StateFlow<Boolean> =
+        userPreferences
+            .scrobbling()
+            .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), true)
+
     fun selectSource(id: Long) {
         viewModelScope.launch { sourceRepository.selectSource(id) }
     }
@@ -71,4 +76,8 @@ class SettingsViewModel(
     fun setSyncConcurrency(value: Int) {
         viewModelScope.launch { userPreferences.setSyncConcurrency(value) }
     }
+
+    fun setScrobbling(enabled: Boolean) {
+        viewModelScope.launch { userPreferences.setScrobbling(enabled) }
+    }
 }