Added serialization plugin

app/build.gradle

@@ -1,7 +1,10 @@
-apply plugin: 'com.android.application'
-apply plugin: 'kotlin-android'
-apply plugin: 'kotlin-android-extensions'
-apply plugin: 'kotlin-kapt'
+plugins {
+    id 'com.android.application'
+    id 'kotlin-android'
+    id 'kotlin-android-extensions'
+    id 'kotlin-kapt'
+    id 'org.jetbrains.kotlin.plugin.serialization' version "$kotlin_version"
+}
 
 def keystorePropertiesFile = rootProject.file("keystore.properties")
 def keystoreProperties = new Properties()
@@ -10,7 +13,7 @@ if (keystorePropertiesFile.exists()) {
 }
 
 android {
-    compileSdkVersion 33
+    compileSdk 33
 
     defaultConfig {
         applicationId "com.simplemobiletools.notes.pro"
@@ -70,4 +73,6 @@ dependencies {
     kapt 'androidx.room:room-compiler:2.5.1'
     implementation 'androidx.room:room-runtime:2.5.1'
     annotationProcessor 'androidx.room:room-compiler:2.5.1'
+
+    implementation "org.jetbrains.kotlinx:kotlinx-serialization-json:1.4.1"
 }

app/proguard-rules.pro

@@ -1,3 +1,27 @@
 -keep class com.simplemobiletools.notes.pro.models.* {
     <fields>;
 }
+
+# Keep `Companion` object fields of serializable classes.
+# This avoids serializer lookup through `getDeclaredClasses` as done for named companion objects.
+-if @kotlinx.serialization.Serializable class **
+-keepclassmembers class <1> {
+    static <1>$Companion Companion;
+}
+
+# Keep `serializer()` on companion objects (both default and named) of serializable classes.
+-if @kotlinx.serialization.Serializable class ** {
+    static **$* *;
+}
+-keepclassmembers class <2>$<3> {
+    kotlinx.serialization.KSerializer serializer(...);
+}
+
+# Keep `INSTANCE.serializer()` of serializable objects.
+-if @kotlinx.serialization.Serializable class ** {
+    public static ** INSTANCE;
+}
+-keepclassmembers class <1> {
+    public static <1> INSTANCE;
+    kotlinx.serialization.KSerializer serializer(...);
+}