library: keep the album grid aligned across a paging reload

A scrobble writes albums.playCount/played, which invalidates the album
list's PagingSource. Room then refreshes from anchor - initialLoadSize/2,
and with placeholders off that page is the presented list: LazyPagingItems
index zero becomes the page's first album, so the grid re-groups its rows
and every cover moves by (oldStart - newStart) % 3 columns, while the last
row's width becomes the loaded count % 3 instead of the library size % 3.

Enable placeholders for the album pager so an album's index is its
absolute position, and draw a placeholder cell for an unloaded slot. The
other lists are single column, so a reload re-anchors them by key and they
never shifted.

This only keeps the grid still; the reloads themselves are still
unnecessary and are tracked separately.

app/src/main/kotlin/com/subtracks/data/repo/LibraryRepository.kt

@@ -174,7 +174,7 @@ class LibraryRepository(
         search: String,
         downloaded: Boolean,
     ): Flow<PagingData<Album>> =
-        pager(40) {
+        pager(40, enablePlaceholders = true) {
             val dao = db.libraryDao()
             if (!downloaded) {
                 dao.albumsPaging(albumsDefaultQuery(sourceId, starred.ordinal, search, sort, descending))
@@ -507,10 +507,11 @@ class LibraryRepository(
 
     private fun <T : Any> pager(
         pageSize: Int,
+        enablePlaceholders: Boolean = false,
         source: () -> PagingSource<Int, T>,
     ): Flow<PagingData<T>> =
         Pager(
-            config = PagingConfig(pageSize = pageSize, enablePlaceholders = false),
+            config = PagingConfig(pageSize = pageSize, enablePlaceholders = enablePlaceholders),
             pagingSourceFactory = source,
         ).flow
 }

app/src/main/kotlin/com/subtracks/ui/library/AlbumsContent.kt

@@ -1,5 +1,6 @@
 package com.subtracks.ui.library
 
+import androidx.compose.foundation.background
 import androidx.compose.foundation.combinedClickable
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
@@ -15,6 +16,7 @@ import androidx.compose.foundation.lazy.grid.GridItemSpan
 import androidx.compose.foundation.lazy.grid.LazyVerticalGrid
 import androidx.compose.foundation.lazy.grid.rememberLazyGridState
 import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.MaterialTheme
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
@@ -43,6 +45,8 @@ import com.subtracks.ui.theme.PrefetchArtworkSeeds
 
 const val ALBUM_COVER_TAG = "album-cover"
 
+const val ALBUM_PLACEHOLDER_TAG = "album-placeholder"
+
 @Composable
 fun AlbumsContent(
     items: LazyPagingItems<Album>,
@@ -117,6 +121,16 @@ fun AlbumsContent(
                                 scrim = true,
                             )
                         }
+                    } else {
+                        Box(
+                            modifier =
+                                Modifier
+                                    .fillMaxWidth()
+                                    .aspectRatio(1f)
+                                    .clip(RoundedCornerShape(2.dp))
+                                    .background(MaterialTheme.colorScheme.surfaceVariant)
+                                    .testTag(ALBUM_PLACEHOLDER_TAG),
+                        )
                     }
                 }
                 item(span = { GridItemSpan(maxLineSpan) }) {