Fix serialization import and quieten CI build logs

- Transport.kt used the reified Json.encodeToString extension without
  importing kotlinx.serialization.encodeToString, so it resolved to the
  two-argument member overload and failed to compile.
- Opt in to ExperimentalMaterial3Api for the clickable game tile.
- Drop --stacktrace from the CI build. A one-line Kotlin error was being
  buried under 150 lines of Gradle internals, which made the actual
  message hard to find in the log.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_015qjfMiPUDshdKEUa6vxqzN

app/src/main/java/org/prolibertate/games/net/Transport.kt

@@ -8,6 +8,7 @@ import kotlinx.coroutines.flow.SharedFlow
 import kotlinx.coroutines.isActive
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
+import kotlinx.serialization.encodeToString
 import java.io.BufferedReader
 import java.io.BufferedWriter
 import java.io.Closeable

app/src/main/java/org/prolibertate/games/ui/AppRoot.kt

@@ -192,6 +192,7 @@ fun MainMenuScreen(
     }
 }
 
+@OptIn(ExperimentalMaterial3Api::class)
 @Composable
 private fun GameTile(game: GameDescriptor, onClick: () -> Unit) {
     Card(