feat: Implement forced logout on app restart and prioritize last logged public key in UnlockScreen

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

@@ -53,7 +53,9 @@ class MainActivity : ComponentActivity() {
             var accountInfoList by remember { mutableStateOf<List<AccountInfo>>(emptyList()) }
             
             // Check for existing accounts and build AccountInfo list
+            // Also force logout so user always sees unlock screen on app restart
             LaunchedEffect(Unit) {
+                accountManager.logout() // Always start logged out
                 val accounts = accountManager.getAllAccounts()
                 hasExistingAccount = accounts.isNotEmpty()
                 accountInfoList = accounts.map { account ->