Make GalleryUiState derivations lazy and drop dead sorts

copy() ran on every state update (each drag-select step included) and eagerly
recomputed five derived properties: two unused O(N log N) sorts, an O(N)
selection filter, an O(N) selectable-URI set and its containsAll check.

- Remove sortedPhotos/sortedSubfolders: never read (the screen sorts with its
  own remember() calls)
- selectedPhotos/selectablePhotoUris/allPhotosSelected become by lazy:
  computed at most once per instance, only when actually read; body delegates
  stay out of generated copy()/equals(), @Immutable semantics hold
- Memoize selectablePhotoUris across drag-select steps in the ViewModel keyed
  by photos list identity, so a drag step no longer recomputes the O(N) set

app/src/main/java/com/example/onecamera/ui/gallery/GalleryViewModel.kt

@@ -95,22 +95,20 @@ data class GalleryUiState(
     val error: String? = null,
 ) {
     val isSelectionMode: Boolean = selectedPhotoUris.isNotEmpty()
-    val selectedPhotos: List<Photo> = photos.filter { it.uri.toString() in selectedPhotoUris }
-    val sortedSubfolders: List<FolderNode> = if (sortAscending) {
-        subfolders.sortedBy { it.name.lowercase() }
-    } else {
-        subfolders.sortedByDescending { it.name.lowercase() }
+
+    val selectedPhotos: List<Photo> by lazy {
+        photos.filter { it.uri.toString() in selectedPhotoUris }
     }
-    val sortedPhotos: List<Photo> = if (sortAscending) {
-        photos.sortedBy { it.name.lowercase() }
-    } else {
-        photos.sortedByDescending { it.name.lowercase() }
-    }
-    val selectablePhotoUris: Set<String> = photos
+
+    val selectablePhotoUris: Set<String> by lazy {
+        photos.asSequence()
             .filter { it.kind == GalleryFileKind.Photo }
-        .map { it.uri.toString() }
-        .toSet()
-    val allPhotosSelected: Boolean = selectablePhotoUris.isNotEmpty() && selectedPhotoUris.containsAll(selectablePhotoUris)
+            .mapTo(HashSet()) { it.uri.toString() }
+    }
+
+    val allPhotosSelected: Boolean by lazy {
+        selectablePhotoUris.isNotEmpty() && selectedPhotoUris.containsAll(selectablePhotoUris)
+    }
 }
 
 @HiltViewModel
@@ -342,18 +340,31 @@ class GalleryViewModel @Inject constructor(
 
     fun clearSelection() {
         dragSelectionBaseline = emptySet()
+        dragSelectableCache = null
         uiState.value = uiState.value.copy(selectedPhotoUris = emptySet())
     }
 
     private var dragSelectionBaseline: Set<String> = emptySet()
 
+    private var dragSelectableCache: Pair<List<Photo>, Set<String>>? = null
+
+    private fun currentSelectablePhotoUris(): Set<String> {
+        val photos = uiState.value.photos
+        dragSelectableCache?.let { (cachedPhotos, uris) ->
+            if (cachedPhotos === photos) return uris
+        }
+        val uris = uiState.value.selectablePhotoUris
+        dragSelectableCache = photos to uris
+        return uris
+    }
+
     fun beginDragSelection(anchorUri: String) {
         dragSelectionBaseline = uiState.value.selectedPhotoUris + anchorUri
         uiState.value = uiState.value.copy(selectedPhotoUris = dragSelectionBaseline)
     }
 
     fun updateDragSelection(rangeUris: Set<String>) {
-        val selectable = uiState.value.selectablePhotoUris
+        val selectable = currentSelectablePhotoUris()
         uiState.value = uiState.value.copy(
             selectedPhotoUris = dragSelectionBaseline + rangeUris.intersect(selectable),
         )
@@ -361,6 +372,7 @@ class GalleryViewModel @Inject constructor(
 
     fun endDragSelection() {
         dragSelectionBaseline = emptySet()
+        dragSelectableCache = null
     }
 
     fun zipSelectedPhotosAndShare() {