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/test/kotlin/com/subtracks/data/repo/DownloadRepositoryTest.kt

@@ -52,6 +52,7 @@ class DownloadRepositoryTest {
     private lateinit var repository: DownloadRepository
     private lateinit var artwork: ArtworkStore
     private lateinit var dir: File
+    private val prefs = fakeUserPreferences()
     private val messages = CopyOnWriteArrayList<String>()
     private val requestedArt = CopyOnWriteArrayList<String>()
     private val artFetchedAfterTheEngineRequest = CopyOnWriteArrayList<Boolean>()
@@ -73,7 +74,7 @@ class DownloadRepositoryTest {
                 .setDriver(BundledSQLiteDriver())
                 .build()
         artwork = ArtworkStore(File(context.cacheDir, "art-${System.nanoTime()}"))
-        sources = SourceRepository(db, OkHttpClient(), fakeUserPreferences(), artwork)
+        sources = SourceRepository(db, OkHttpClient(), prefs, artwork)
         engine = FakeDownloadEngine()
         dir = File(context.cacheDir, "downloads-${System.nanoTime()}")
         repository =
@@ -956,6 +957,29 @@ class DownloadRepositoryTest {
             assertNull(repository.localUri("s1"))
         }
 
+    @Test
+    fun downloadsAreWifiOnlyUntilThePreferenceAllowsMetered() =
+        runTest {
+            seedLibrary()
+            runBlocking { repository.download(1, "s1") }
+            assertFalse(
+                engine.requests
+                    .single()
+                    .second.allowMetered,
+            )
+
+            prefs.setDownloadOverMetered(true)
+            await { sources.downloadsAllowedOverMetered() }
+
+            runBlocking { repository.download(1, "s2") }
+
+            assertTrue(
+                engine.requests
+                    .last()
+                    .second.allowMetered,
+            )
+        }
+
     private fun requestedSongIds(): List<String> = engine.requests.map { it.second.path.substringAfterLast('/') }
 
     private fun seedSongs(tracks: IntRange) {

app/src/test/kotlin/com/subtracks/ui/AppScreenshotTest.kt

@@ -344,12 +344,14 @@ class AppScreenshotTest {
                     mobileQuality = StreamQuality(192, "mp3"),
                     syncConcurrency = 4,
                     downloadQuality = StreamQuality(128, "opus"),
+                    downloadOverMetered = false,
                     onSelectSource = {},
                     onEditServer = {},
                     onWifiQualityChange = {},
                     onMobileQualityChange = {},
                     onSyncConcurrencyChange = {},
                     onDownloadQualityChange = {},
+                    onDownloadOverMeteredChange = {},
                     onAddServer = {},
                     onOpenDownloads = {},
                     onBack = {},