fix(conversations): don't show the fallback avatar while loading

Scrolling the conversation list flashed the generic fallback avatar on
rows entering the viewport for the first time in a session: the lazy
list disposes off-screen rows, and re-entering rows without a memory
cache entry showed the fallback as a de-facto loading indicator until
the disk read or fetch completed.

Render nothing while an avatar loads (the slot keeps its size and the
avatar crossfades in), keep the last shown avatar as the placeholder
when one is known, and reserve the fallback icon for what its name
says: a failed load with no previously shown avatar.

Assisted-by: Claude Code:claude-fable-5
Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

Author: AndyScherzinger

app/src/main/java/com/nextcloud/talk/conversationlist/ui/ConversationListItem.kt

@@ -269,14 +269,13 @@ private fun ConversationAvatarImage(model: ConversationModel, currentUser: User,
                 val lastShownAvatar = remember(request) {
                     imageLoader.memoryCache?.get(aliasKey)?.bitmap?.let { BitmapPainter(it.asImageBitmap()) }
                 }
-                val fallback = painterResource(R.drawable.account_circle_96dp)
                 AsyncImage(
                     model = request,
                     imageLoader = imageLoader,
                     contentDescription = stringResource(R.string.avatar),
                     contentScale = ContentScale.Crop,
-                    placeholder = lastShownAvatar ?: fallback,
-                    error = lastShownAvatar ?: fallback,
+                    placeholder = lastShownAvatar,
+                    error = lastShownAvatar ?: painterResource(R.drawable.account_circle_96dp),
                     modifier = modifier
                 )
             }