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/system/SystemScreen.kt

@@ -786,7 +786,7 @@ private fun LazyListScope.gatewaySection(
                                         style = MaterialTheme.typography.bodyMedium,
                                         modifier = Modifier.weight(1f),
                                     )
-                                    platform.state?.let { platformState ->
+                                    platform?.state?.let { platformState ->
                                         StatusBadge(
                                             text = platformState,
                                             status =