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/markdown.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="MarkdownSettings">
+    <option name="previewPanelProviderInfo">
+      <ProviderInfo name="Compose (experimental)" className="com.intellij.markdown.compose.preview.ComposePanelProvider" />
+    </option>
+  </component>
+</project>