Add decode failure logging to DenseThumb to diagnose sparse thumbnails

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
@@ -182,9 +184,11 @@ private fun DenseThumb(
                     pullFn = pullFn,
                 )
             }
-            if (bytes != null) {
+            if (bytes != null && bytes.isNotEmpty()) {
                 runCatching {
                     thumbnail = SkiaImage.makeFromEncoded(bytes).toComposeImageBitmap()
+                }.onFailure { e ->
+                    thumbLog.warn("Decode failed for ${photo.filename} (${bytes.size} bytes): ${e.message}")
                 }
             }
         }