fix Live Call: include type field in WS protocol messages

encodeDefaults=false caused kotlinx.serialization to skip the
type="setup" field since it was a default value. The server's
parseClientMessage needs the type field to dispatch — without it
every setup message was rejected as "malformed_message".

Changed to encodeDefaults=true + explicitNulls=false so type is
always serialized but null optionals are still omitted.

app/src/main/java/dev/kaizen/app/live/LiveProtocol.kt

@@ -14,7 +14,7 @@ import kotlinx.serialization.json.jsonPrimitive
 import kotlinx.serialization.json.long
 import kotlinx.serialization.json.longOrNull
 
-private val json = Json { ignoreUnknownKeys = true; encodeDefaults = false }
+private val json = Json { ignoreUnknownKeys = true; encodeDefaults = true; explicitNulls = false }
 
 // ─── Client → Server ──────────────────────────────────────────────────────