Fix photo viewer flicker when opening an image

- Keep the grid composed under the viewer instead of swapping it for a
  black box, removing the momentary black flash while the pager draws
  its first frame after tapping a photo
- Restore the pre-audit fade: thumbnail stays fully opaque under the
  fading-in full image instead of crossfading both over black

app/src/main/java/com/example/onecamera/ui/gallery/GalleryScreen.kt

@@ -49,8 +49,8 @@ import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.ExperimentalComposeUiApi
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.alpha
 import androidx.compose.ui.graphics.Color
-import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.layout.ContentScale
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.res.painterResource
@@ -408,14 +408,6 @@ fun GalleryScreen(
                 Box(modifier = Modifier.fillMaxSize()) {
                     when {
 
-                        uiState.selectedPhoto != null -> {
-                            Box(
-                                modifier = Modifier
-                                    .fillMaxSize()
-                                    .background(Color.Black),
-                            )
-                        }
-
                         !uiState.viewPreferencesLoaded -> {
                             Box(modifier = Modifier.fillMaxSize())
                         }
@@ -670,16 +662,16 @@ private fun ZoomablePhoto(
 
     Box(modifier = modifier.fillMaxSize()) {
 
+        if (fullAlpha < 1f) {
             AsyncImage(
                 model = remember(photo.uri, context, fadeMs) {
                     thumbnailDraftRequest(context, photo.uri, crossfadeMillis = fadeMs)
                 },
                 contentDescription = null,
                 contentScale = ContentScale.Fit,
-            modifier = Modifier
-                .fillMaxSize()
-                .graphicsLayer { alpha = 1f - fullAlpha },
+                modifier = Modifier.fillMaxSize(),
             )
+        }
         CoilZoomAsyncImage(
             model = fullImageRequest,
             contentDescription = photo.name,
@@ -688,7 +680,7 @@ private fun ZoomablePhoto(
             onError = { isFullReady = false },
             modifier = Modifier
                 .fillMaxSize()
-                .graphicsLayer { alpha = fullAlpha },
+                .alpha(fullAlpha),
             zoomState = zoomState,
         )
     }