Preload next image during slideshow (#1847)

## Description
When viewing images in a slideshow, this PR enqueues the next image so
it will likely be cached and ready to display when the slideshow
advances.

This works for both automatic and manual scrolling as well as advancing
next or previous images.

### Related issues
Closes #1835 

### Testing
Emulator with large photos

## Screenshots
N/A

## AI or LLM usage
None

app/src/main/java/com/github/damontecres/wholphin/ui/slideshow/SlideshowViewModel.kt

@@ -6,6 +6,9 @@ import androidx.compose.runtime.Stable
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
 import androidx.media3.common.Player
+import coil3.imageLoader
+import coil3.request.ImageRequest
+import coil3.size.Size
 import com.github.damontecres.wholphin.data.ChosenStreams
 import com.github.damontecres.wholphin.data.PlaybackEffectDao
 import com.github.damontecres.wholphin.data.ServerRepository
@@ -172,7 +175,7 @@ class SlideshowViewModel
             val size = state.value.items.size
             val newPosition = state.value.position + 1
             return if (newPosition < size) {
-                updatePosition(newPosition)
+                updatePosition(newPosition, true)
                 true
             } else {
                 false
@@ -182,14 +185,23 @@ class SlideshowViewModel
         fun previousImage(): Boolean {
             val newPosition = state.value.position - 1
             return if (newPosition >= 0) {
-                updatePosition(newPosition)
+                updatePosition(newPosition, false)
                 true
             } else {
                 false
             }
         }
 
-        fun updatePosition(position: Int): Job? =
+        /**
+         * Update image state to the specified position
+         *
+         * @param position the index to use
+         * @param isNext whether moving to the next or previous image for prefetching purposes
+         */
+        fun updatePosition(
+            position: Int,
+            isNext: Boolean = true,
+        ): Job? =
             (state.value.items as? ApiRequestPager<*>)?.let { pager ->
                 viewModelScope.launchIO {
                     try {
@@ -286,6 +298,31 @@ class SlideshowViewModel
                             )
                         }
                     }
+                    try {
+                        // If there is a next/previous image, fetch it into the image cache
+                        val nextIndex = position + if (isNext) 1 else -1
+                        if (nextIndex in pager.indices) {
+                            pager.getBlocking(nextIndex)?.let { nextImage ->
+                                if (nextImage.data.mediaType == MediaType.PHOTO) {
+                                    val url = api.libraryApi.getDownloadUrl(nextImage.id)
+                                    val request =
+                                        ImageRequest
+                                            .Builder(context)
+                                            .data(url)
+                                            .size(Size.ORIGINAL)
+                                            .build()
+                                    context.imageLoader.enqueue(request)
+                                    Timber.v(
+                                        "Prefetching index=%s, id=%s",
+                                        nextIndex,
+                                        nextImage.id,
+                                    )
+                                }
+                            }
+                        }
+                    } catch (ex: Exception) {
+                        Timber.e(ex, "Error preloading image")
+                    }
                 }
             }
 
@@ -339,7 +376,7 @@ class SlideshowViewModel
         fun pulseSlideshow() = pulseSlideshow(slideshowDelay)
 
         fun pulseSlideshow(milliseconds: Long) {
-            Timber.v("pulseSlideshow $milliseconds")
+//            Timber.v("pulseSlideshow $milliseconds")
             slideshowJob?.cancel()
             slideshowJob =
                 viewModelScope