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/migrations.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectMigrations">
+    <option name="MigrateToGradleLocalJavaHome">
+      <set>
+        <option value="$PROJECT_DIR$" />
+      </set>
+    </option>
+  </component>
+</project>