Avatars merge instead of replacing on refresh

Same rule as iOS: the live refresh replaced the whole map, so a refresh
while the buddy list was briefly empty produced an empty map that blanked
every picture. Merge, and ignore an empty result.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/kotlin/io/flamenet/messenger/app/MessengerViewModel.kt

@@ -1750,7 +1750,17 @@ class MessengerViewModel(app: Application) : AndroidViewModel(app) {
                                 current.avatars(signedIn()?.buddies?.map { it.userId }.orEmpty())
                             }.getOrNull()
                         }
-                        fresh?.let { live -> update { it.copy(avatars = live) } }
+                        // 🔴 **Merge, never replace.** A refresh made while the
+                        // buddy list was momentarily empty — a roster fetch in
+                        // flight, or the list loading from the vault — produced
+                        // an empty map that wiped every picture from the UI at
+                        // once. The avatars were in the vault the whole time,
+                        // which is why "the profile pictures are gone" looked
+                        // like data loss rather than a render bug. Absent means
+                        // "not loaded yet", never "deleted".
+                        fresh?.takeIf { it.isNotEmpty() }?.let { live ->
+                            update { it.copy(avatars = it.avatars + live) }
+                        }
                     }
                     if (threads.isNotEmpty()) {
                         // Whoever just wrote may not be on the list at all —