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/app/src/main/res/values/strings.xml

@@ -0,0 +1,7 @@
+<?xml version='1.0' encoding='utf-8'?>
+<resources>
+    <string name="app_name">Slim Budget</string>
+    <string name="title_activity_main">Slim Budget</string>
+    <string name="package_name">kr.sblog.slimbudget</string>
+    <string name="custom_url_scheme">kr.sblog.slimbudget</string>
+</resources>