player: keep the thumbnail until the first frame, not just STATE_READY

ExoPlayer reports STATE_READY a beat before it actually draws the first frame,
so the buffering thumbnail was removed into a black gap right before playback
("thumbnail, then black, then playing"). Hold the thumbnail overlay until
onRenderedFirstFrame fires, so it hands straight over to the picture.

app/src/main/java/net/jeena/jtubetv/ui/screens/PlayerScreen.kt

@@ -323,6 +323,10 @@ private fun PlayerSurface(
     // 200 ms: quick flaps cancel the pending change and the overlay stays
     // composed, so the animation never restarts mid-playback.
     var bufferingPending by remember { mutableStateOf(true) }
+    // STATE_READY can fire before the first frame is actually drawn, leaving a black
+    // gap between the thumbnail and the picture. Keep the thumbnail until the first
+    // frame really renders (user report: "thumbnail, then black, then playing").
+    var firstFrame by remember { mutableStateOf(false) }
     LaunchedEffect(bufferingPending) {
         kotlinx.coroutines.delay(200)
         buffering = bufferingPending
@@ -436,6 +440,10 @@ private fun PlayerSurface(
             override fun onPlaybackStateChanged(playbackState: Int) {
                 bufferingPending = playbackState != Player.STATE_READY
             }
+
+            override fun onRenderedFirstFrame() {
+                firstFrame = true
+            }
         })
         onDispose {
             viewModel.persistExit(shortUuid, (player.currentPosition / 1000).toInt())
@@ -588,7 +596,7 @@ private fun PlayerSurface(
         // ready to render — metadata resolution alone (the Loading state) is not
         // enough (issue #8). Fullscreen loading shows the video's own thumbnail
         // dimmed under the circle loader (issue #19) instead of plain black.
-        if (buffering) {
+        if (buffering || !firstFrame) {
             Box(
                 Modifier
                     .fillMaxSize()