refactor: migrate Gson serialization to kotlinx.serialization (#475)

* refactor: migrate Gson serialization to kotlinx.serialization

* fix(dto): resolve serializing Any? and Map<String, Any?> with JsonElement

* test(refactor): fix unit test issues under kotlinx.serialization and update nullable DTO defaults

* fix(chat): support flexible numeric and string decoding for message timestamps

* fix(chat): sequentialize cache loading to avoid overwriting network state

app/src/main/java/com/m57/hermescontrol/ui/gateway/GatewayScreen.kt

@@ -267,7 +267,7 @@ fun GatewayScreen(
                                                         style = MaterialTheme.typography.bodyLarge,
                                                         fontWeight = FontWeight.Medium,
                                                     )
-                                                    val stateText = pStatus.state?.uppercase() ?: "UNKNOWN"
+                                                    val stateText = pStatus?.state?.uppercase() ?: "UNKNOWN"
                                                     val badgeColor =
                                                         when (stateText) {
                                                             "RUNNING" -> Color(0xFF4CAF50)