multiplatform: fix image loading performance and layout stability (#6631)

- Replace runBlocking { imageAndFilePath(file) } with LaunchedEffect +
  withContext(Dispatchers.IO) to unblock main thread on all platforms
- Set fixed container size (width + aspectRatio) from preview bitmap to
  eliminate layout shifts during async image loading
- Cache base64ToBitmap() with remember() in CIImageView and FramedItemView
- Desktop: replace imageBitmap.toAwtImage().toPainter() with BitmapPainter
  to eliminate unnecessary round-trip conversion
- Desktop: add LRU cache for base64ToBitmap (200 entries) and
  getLoadedImage (30 entries) to survive LazyColumn item disposal
- Clear loaded image cache on app file deletion via expect/actual

Co-authored-by: Evgeny Poberezkin <evgeny@poberezkin.com>

apps/multiplatform/common/src/androidMain/kotlin/chat/simplex/common/views/helpers/Utils.android.kt

@@ -182,6 +182,8 @@ private fun spannableStringToAnnotatedString(
 actual fun getAppFileUri(fileName: String): URI =
   FileProvider.getUriForFile(androidAppContext, "$APPLICATION_ID.provider", if (File(fileName).isAbsolute) File(fileName) else File(getAppFilePath(fileName))).toURI()
 
+actual fun clearImageCaches() {}
+
 // https://developer.android.com/training/data-storage/shared/documents-files#bitmap
 actual suspend fun getLoadedImage(file: CIFile?): Pair<ImageBitmap, ByteArray>? {
   val filePath = getLoadedFilePath(file)