Fix scroll-back placeholders: in-memory bitmap cache in DensePhotoGrid

bitmapCache (MutableMap keyed on devicePath) lives at DensePhotoGrid
level and survives Compose cell recycling. DenseThumb checks memory
cache first — scroll-back is now instant for already-loaded cells.
Disk cache (ThumbnailCache) is only hit when bitmap not in memory.

src/main/kotlin/com/bolenpad/photophetch/ui/DensePhotoGrid.kt

@@ -53,6 +53,11 @@ fun DensePhotoGrid(
     onDeselectAllFolder: (List<PhonePhoto>) -> Unit,
     modifier: Modifier = Modifier,
 ) {
+    // In-memory bitmap cache — survives cell recycling during scroll
+    // keyed on cacheKey so it resets when device changes
+    val bitmapCache = remember(cacheKey) {
+        mutableMapOf<String, ImageBitmap>()
+    }
     if (folderSections.isEmpty() || folderSections.all { it.allPhotos.isEmpty() }) {
         Box(modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
             Column(
@@ -144,6 +149,7 @@ fun DensePhotoGrid(
                             cacheKey = cacheKey,
                             pullFn = pullFn,
                             thumbnailVersion = thumbnailVersion,
+                            bitmapCache = bitmapCache,
                             onToggle = { onTogglePhoto(labeled.photo) },
                             dateLabel = labeled.dateLabel,
                             modifier = Modifier.weight(1f),
@@ -167,13 +173,16 @@ private fun DenseThumb(
     cacheKey: String,
     pullFn: suspend (devicePath: String, localDestPath: Path) -> Boolean,
     thumbnailVersion: Int,
+    bitmapCache: MutableMap<String, ImageBitmap>,
     onToggle: () -> Unit,
     dateLabel: String?,
     modifier: Modifier = Modifier,
 ) {
-    var thumbnail by remember(photo.devicePath) { mutableStateOf<ImageBitmap?>(null) }
+    // Check in-memory cache first (survives Compose cell recycling during scroll)
+    var thumbnail by remember(photo.devicePath) {
+        mutableStateOf(bitmapCache[photo.devicePath])
+    }
 
-    // Re-run when thumbnailVersion increments (new batch cached) or on first composition
     LaunchedEffect(photo.devicePath, thumbnailVersion) {
         if (thumbnail == null) {
             val bytes = withContext(Dispatchers.IO) {
@@ -186,7 +195,9 @@ private fun DenseThumb(
             }
             if (bytes != null && bytes.isNotEmpty()) {
                 runCatching {
-                    thumbnail = SkiaImage.makeFromEncoded(bytes).toComposeImageBitmap()
+                    val bitmap = SkiaImage.makeFromEncoded(bytes).toComposeImageBitmap()
+                    bitmapCache[photo.devicePath] = bitmap  // store in memory cache
+                    thumbnail = bitmap
                 }.onFailure { e ->
                     thumbLog.warn("Decode failed for ${photo.filename} (${bytes.size} bytes): ${e.message}")
                 }