feat: Update profile saving logic to follow desktop version pattern and enhance local data handling

app/src/main/java/com/rosetta/messenger/MainActivity.kt

@@ -439,10 +439,11 @@ fun MainScreen(
     val privateKeyHash = account?.privateKeyHash ?: ""
     
     // Username state - загружается из EncryptedAccount
+    // Following desktop version pattern: username is stored locally and loaded on app start
     var accountUsername by remember { mutableStateOf("") }
     var reloadTrigger by remember { mutableIntStateOf(0) }
     
-    // Load username from AccountManager
+    // Load username from AccountManager (persisted in DataStore)
     val context = LocalContext.current
     LaunchedEffect(accountPublicKey, reloadTrigger) {
         if (accountPublicKey.isNotBlank() && accountPublicKey != "04c266b98ae5") {
@@ -706,7 +707,10 @@ fun MainScreen(
                         accountPrivateKeyHash = privateKeyHash,
                         onBack = { showProfileScreen = false },
                         onSaveProfile = { name, username ->
-                            // Update username state and trigger reload from DB
+                            // Following desktop version pattern:
+                            // 1. Server confirms save (handled in ProfileViewModel)
+                            // 2. Local DB updated (handled in ProfileScreen LaunchedEffect)
+                            // 3. This callback updates UI state (reloads username from DB)
                             accountUsername = username
                             reloadTrigger++
                             Log.d("MainActivity", "Profile saved: name=$name, username=$username, reloading...")