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>

package.json

@@ -6,12 +6,19 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
+    "build:app": "vite build --mode capacitor",
+    "cap:sync": "npm run build:app && cap sync",
+    "cap:android": "cap open android",
     "preview": "vite preview",
     "lint": "run-s lint:*",
     "lint:oxlint": "oxlint . --fix",
     "lint:eslint": "eslint . --fix --cache"
   },
   "dependencies": {
+    "@capacitor/android": "^8.3.4",
+    "@capacitor/app": "^8.1.0",
+    "@capacitor/cli": "^8.3.4",
+    "@capacitor/core": "^8.3.4",
     "@toast-ui/editor": "^3.2.2",
     "@toast-ui/editor-plugin-code-syntax-highlight": "^3.1.0",
     "axios": "^1.16.1",