all: refactor model classes and serialization

- Move model classes to a standalone `model` Gradle module
- Migrate from Moshi to kotlinx.serialization for multiplatform support

model/build.gradle.kts

@@ -0,0 +1,11 @@
+plugins {
+  kotlin("multiplatform")
+  kotlin("plugin.serialization")
+}
+
+kotlin {
+  jvm { compilations.all { kotlinOptions.jvmTarget = "11" } }
+  sourceSets["commonMain"].apply {
+    dependencies { implementation(libs.kotlinx.serialization.core) }
+  }
+}