feat: 안드로이드 하이브리드 앱(Capacitor) 구성

- capacitor.config + android/ 네이티브 프로젝트
- 앱 전용 빌드(.env.capacitor, build:app), 뒤로가기·safe-area
- docs/ANDROID.md

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

android/capacitor.settings.gradle

@@ -0,0 +1,6 @@
+// DO NOT EDIT THIS FILE! IT IS GENERATED EACH TIME "capacitor update" IS RUN
+include ':capacitor-android'
+project(':capacitor-android').projectDir = new File('../node_modules/@capacitor/android/capacitor')
+
+include ':capacitor-app'
+project(':capacitor-app').projectDir = new File('../node_modules/@capacitor/app/android')