wire the sync fetch concurrency to a Settings option

The bound was only a constructor default with no production consumer. Add a
"Sync concurrency" preference (Settings -> Network, default 4) and pass it
through SourceRepository to SubsonicSource, so sync parallelism is tunable
without a rebuild. Both the app-scoped source and the on-demand
`activeMusicSource()` used by sync read the preference.

- UserPreferences.syncConcurrency()/setSyncConcurrency() (clamped >= 1)
- SourceRepository combines the preference into toMusicSource
- Settings row + dialog; SettingsViewModel state and setter
- Preferences test for default/round-trip/clamp

app/src/test/kotlin/com/subtracks/data/prefs/UserPreferencesTest.kt

@@ -58,6 +58,24 @@ class UserPreferencesTest {
             file.delete()
         }
 
+    @Test
+    fun syncConcurrencyDefaultsAndClampsToAtLeastOne() =
+        runTest {
+            val file = File.createTempFile("user-prefs", ".preferences_pb").apply { delete() }
+            val store = PreferenceDataStoreFactory.create(scope = backgroundScope) { file }
+            val prefs = UserPreferences(store)
+
+            assertEquals(DEFAULT_SYNC_CONCURRENCY, prefs.syncConcurrency().first())
+
+            prefs.setSyncConcurrency(8)
+            assertEquals(8, prefs.syncConcurrency().first())
+
+            prefs.setSyncConcurrency(0)
+            assertEquals(1, prefs.syncConcurrency().first())
+
+            file.delete()
+        }
+
     @Test
     fun lastSeedRoundTripsWithAndWithoutASecondary() =
         runTest {

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

@@ -306,10 +306,12 @@ class AppScreenshotTest {
                     activeSourceId = 1,
                     wifiQuality = StreamQuality(),
                     mobileQuality = StreamQuality(192, "mp3"),
+                    syncConcurrency = 4,
                     onSelectSource = {},
                     onDeleteSource = {},
                     onWifiQualityChange = {},
                     onMobileQualityChange = {},
+                    onSyncConcurrencyChange = {},
                     onAddServer = {},
                     onBack = {},
                 )