playback: prepare the restored queue so the session starts on launch

Restoring the saved queue left the player idle, so the media session had no
notification and Bluetooth or Automate saw no active player until playback
began. Prepare the restored window without autoplaying, which surfaces the
paused session when the app opens. A queue from another source stays stopped,
and an offline queue without downloads has no window to prepare.

Closes austinried/subtracks#202

app/src/main/kotlin/com/subtracks/playback/PlaybackController.kt

@@ -1070,6 +1070,7 @@ class PlaybackController(
             }
             lastSavedPositionMs = savedPositionMs
             loadWindow(anchor.position, autoplay = false, startPositionMs = savedPositionMs)
+            if ((player?.itemCount ?: 0) > 0) player?.ensurePrepared()
         }
 
     private suspend fun stop() = startLock.withLock { stopLocked() }

app/src/test/kotlin/com/subtracks/playback/PlaybackControllerTest.kt

@@ -343,7 +343,46 @@ class PlaybackControllerTest {
     }
 
     @Test
-    fun togglingPlayOnARestoredQueuePreparesBeforePlaying() {
+    fun restoringAPreviousQueuePreparesTheSessionWithoutPlaying() {
+        seedAlbum(3, sourceId = 1)
+        runBlocking { queues.replace(listOf(queues.albumEntry(1, "al1"))) }
+
+        controller.connect()
+        await { handle.items.isNotEmpty() }
+
+        assertTrue(handle.operations.contains("prepare"))
+        assertFalse(handle.operations.contains("play"))
+        assertFalse(handle.isIdle)
+        assertFalse(handle.playWhenReady)
+        assertFalse(controller.state.value.isPlaying)
+    }
+
+    @Test
+    fun restoringAnEmptyQueueDoesNotPrepare() {
+        seedAlbum(3, sourceId = 1)
+
+        controller.connect()
+        await { controller.ready.value }
+
+        assertTrue(handle.items.isEmpty())
+        assertFalse(handle.operations.contains("prepare"))
+    }
+
+    @Test
+    fun restoringAnOfflineQueueWithoutDownloadsDoesNotPrepare() {
+        seedAlbum(3, sourceId = 1)
+        runBlocking { queues.replace(listOf(queues.albumEntry(1, "al1"))) }
+        setOffline()
+
+        controller.connect()
+        await { controller.ready.value }
+
+        assertTrue(handle.items.isEmpty())
+        assertFalse(handle.operations.contains("prepare"))
+    }
+
+    @Test
+    fun togglingPlayOnARestoredQueuePlaysThePreparedWindow() {
         seedAlbum(5, sourceId = 1)
         runBlocking { queues.replace(listOf(queues.albumEntry(1, "al1"))) }
 
@@ -354,7 +393,8 @@ class PlaybackControllerTest {
         controller.togglePlayPause()
         await { handle.operations.contains("play") }
 
-        assertEquals(listOf("prepare", "play"), handle.operations)
+        assertEquals(listOf("play"), handle.operations)
+        assertTrue(handle.playWhenReady)
     }
 
     @Test