Revert to spec's free web plan + PWA install (iPhone/Android, $0)

명세서 준수: 네이티브(Flutter/Kotlin) 경로 폐기 — 유료 iOS 개발자 등록·맥이 필요해 비용 0 원칙과 명세서에 어긋남.
- app/(Flutter) 전부 제거, 원래 웹 클라이언트 복원.
- PWA 추가: manifest.webmanifest + sw.js(오프라인 캐시) + 앱 아이콘(192/512/apple-touch).
  → iPhone Safari·Android Chrome에서 '홈화면에 추가'로 전체화면 앱처럼 실행, 스토어/맥/등록비 불필요.
- Cloudflare Pages 무료 배포 대상. 서버(Workers) 계획 불변.

app/.gitignore

@@ -1,48 +0,0 @@
-# Miscellaneous
-*.class
-*.log
-*.pyc
-*.swp
-.DS_Store
-.atom/
-.build/
-.buildlog/
-.history
-.svn/
-.swiftpm/
-migrate_working_dir/
-
-# IntelliJ related
-*.iml
-*.ipr
-*.iws
-.idea/
-
-# The .vscode folder contains launch configuration and tasks you configure in
-# VS Code which you may wish to be included in version control, so this line
-# is commented out by default.
-#.vscode/
-
-# Flutter/Dart/Pub related
-**/doc/api/
-**/ios/Flutter/.last_build_id
-.dart_tool/
-.flutter-plugins-dependencies
-.pub-cache/
-.pub/
-/build/
-/coverage/
-
-# Symbolication related
-app.*.symbols
-
-# Obfuscation related
-app.*.map.json
-
-# Android Studio will place build artifacts here
-/android/app/debug
-/android/app/profile
-/android/app/release
-
-# Widget Preview related
-.widget_preview/