let the user allow downloads on mobile data

Downloads asked the platform for an unmetered network and nothing more, so
there was no way to run them on mobile. A setting under Downloads now decides,
defaulting to Wi-Fi only, and it also governs roaming: the platform permits
roaming by default, which would have contradicted the Wi-Fi-only choice.

The flag rides the enqueue request like the download quality does, cached in the
source repository so the poller does not read the preference per row.

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

@@ -38,6 +38,11 @@ class SettingsViewModel(
             .downloadQuality()
             .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), StreamQuality())
 
+    val downloadOverMetered: StateFlow<Boolean> =
+        userPreferences
+            .downloadOverMetered()
+            .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), false)
+
     val syncConcurrency: StateFlow<Int> =
         userPreferences
             .syncConcurrency()
@@ -59,6 +64,10 @@ class SettingsViewModel(
         viewModelScope.launch { userPreferences.setDownloadQuality(quality) }
     }
 
+    fun setDownloadOverMetered(allowed: Boolean) {
+        viewModelScope.launch { userPreferences.setDownloadOverMetered(allowed) }
+    }
+
     fun setSyncConcurrency(value: Int) {
         viewModelScope.launch { userPreferences.setSyncConcurrency(value) }
     }