chore: use src/main/kotlin source directories in :app

Kotlin-only reference repo — move the app's main/test/androidTest source roots
from src/main/java to src/main/kotlin (the core/feature modules already use kotlin).
Verified: :app:compileDebugKotlin produces MainActivity from src/main/kotlin.

.idea/kotlinc.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="KotlinJpsPluginSettings">
+    <option name="externalSystemId" value="Gradle" />
+    <option name="version" value="2.3.20" />
+  </component>
+</project>