refactor: migrate to kotlinx serialization, eliminate gson dependency

app/src/main/java/com/bitcointxoko/gudariwallet/api/JsonProvider.kt

@@ -0,0 +1,23 @@
+package com.bitcointxoko.gudariwallet.api
+
+import kotlinx.serialization.json.Json
+
+/**
+ * Single source of truth for the app's Json instance.
+ *
+ * All serialization/deserialization — Retrofit, DataStore caches,
+ * and WebSocket message parsing — must use this instance so that
+ * configuration is applied consistently everywhere.
+ *
+ * Custom type serializers (e.g. [SuccessActionSerializer],
+ * [FlexibleStringSerializer]) are registered directly on the model
+ * classes via @Serializable(with = ...) so no explicit registration
+ * is needed here.
+ */
+object JsonProvider {
+    val json: Json = Json {
+        ignoreUnknownKeys = true   // tolerate extra fields from the API
+        isLenient = true           // accept unquoted/malformed JSON from some endpoints
+        explicitNulls = false      // omit null fields when serializing requests
+    }
+}