Add decode failure logging to DenseThumb to diagnose sparse thumbnails

When the background batch pre-fetch writes new thumbnails to cache,
thumbnailVersion increments. DenseThumb uses it as a LaunchedEffect key
so cells retry loading from cache after each batch completes.

Also removed thumbFailed permanent gate — thumbnails retry on each
thumbnailVersion change instead of giving up after one failure.

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

@@ -94,6 +94,10 @@ class AppState(private val configStore: ConfigStore) {
     private val _selectedDevice = MutableStateFlow<DeviceInfo?>(null)
     val selectedDevice: StateFlow<DeviceInfo?> = _selectedDevice.asStateFlow()
 
+    /** Incremented after each thumbnail batch completes — DenseThumb observes this to retry */
+    private val _thumbnailVersion = MutableStateFlow(0)
+    val thumbnailVersion: StateFlow<Int> = _thumbnailVersion.asStateFlow()
+
     fun scanForDevices() {
         scope.launch {
             _deviceScan.value = DeviceScanState.Scanning
@@ -240,6 +244,10 @@ class AppState(private val configStore: ConfigStore) {
                     }
                 }
                 loaded += batchLoaded
+                if (batchLoaded > 0) {
+                    // Notify UI that new thumbnails are available
+                    _thumbnailVersion.value++
+                }
                 log.info("iOS thumbnails: $loaded / ${needed.size} loaded (batch ${batchIdx + 1}/${(needed.size + batchSize - 1) / batchSize})")
             } catch (e: Exception) {
                 log.warn("iOS thumbnail batch ${batchIdx + 1} failed: ${e.message}")

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

@@ -92,6 +92,7 @@ fun BrowseScreen(state: AppState) {
 
                     VerticalDivider()
 
+                    val thumbnailVersion by state.thumbnailVersion.collectAsState()
                     DensePhotoGrid(
                         folderSections = folderSections,
                         selectedPhotos = selectedPhotos,
@@ -101,6 +102,7 @@ fun BrowseScreen(state: AppState) {
                                 state.connectorFor(d).pullToFile(d, devicePath, localPath)
                             } ?: false
                         },
+                        thumbnailVersion = thumbnailVersion,
                         columnCount = columnCount,
                         listState = listState,
                         onTogglePhoto = { state.togglePhotoSelection(it) },

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

@@ -31,6 +31,8 @@ import java.time.LocalDate
 import java.time.ZoneId
 import java.time.format.DateTimeFormatter
 
+private val thumbLog = org.slf4j.LoggerFactory.getLogger("DenseThumb")
+
 /**
  * Dense grid: folder headers + flat photo grid per folder.
  * Each photo that is the first of its day gets a small date label
@@ -43,6 +45,7 @@ fun DensePhotoGrid(
     selectedPhotos: Set<PhonePhoto>,
     cacheKey: String,
     pullFn: suspend (devicePath: String, localDestPath: Path) -> Boolean,
+    thumbnailVersion: Int = 0,
     columnCount: Int,
     listState: LazyListState,
     onTogglePhoto: (PhonePhoto) -> Unit,
@@ -140,6 +143,7 @@ fun DensePhotoGrid(
                             isSelected = labeled.photo in selectedPhotos,
                             cacheKey = cacheKey,
                             pullFn = pullFn,
+                            thumbnailVersion = thumbnailVersion,
                             onToggle = { onTogglePhoto(labeled.photo) },
                             dateLabel = labeled.dateLabel,
                             modifier = Modifier.weight(1f),
@@ -162,15 +166,16 @@ private fun DenseThumb(
     isSelected: Boolean,
     cacheKey: String,
     pullFn: suspend (devicePath: String, localDestPath: Path) -> Boolean,
+    thumbnailVersion: Int,
     onToggle: () -> Unit,
     dateLabel: String?,
     modifier: Modifier = Modifier,
 ) {
     var thumbnail by remember(photo.devicePath) { mutableStateOf<ImageBitmap?>(null) }
-    var thumbFailed by remember(photo.devicePath) { mutableStateOf(false) }
 
-    LaunchedEffect(photo.devicePath) {
-        if (!thumbFailed && thumbnail == null) {
+    // Re-run when thumbnailVersion increments (new batch cached) or on first composition
+    LaunchedEffect(photo.devicePath, thumbnailVersion) {
+        if (thumbnail == null) {
             val bytes = withContext(Dispatchers.IO) {
                 ThumbnailCache.get(
                     cacheKey = cacheKey,
@@ -179,12 +184,12 @@ private fun DenseThumb(
                     pullFn = pullFn,
                 )
             }
-            if (bytes != null) {
+            if (bytes != null && bytes.isNotEmpty()) {
                 runCatching {
                     thumbnail = SkiaImage.makeFromEncoded(bytes).toComposeImageBitmap()
-                }.onFailure { thumbFailed = true }
-            } else {
-                thumbFailed = true
+                }.onFailure { e ->
+                    thumbLog.warn("Decode failed for ${photo.filename} (${bytes.size} bytes): ${e.message}")
+                }
             }
         }
     }