feat: demo page switch

frontend/messages/en.json

@@ -59,6 +59,7 @@
     "email_not_exist": "Email address not found",
     "signup_error": "Sign up failed",
     "signin_error": "Sign in failed",
+    "demo_page_warning": "This is a demo environment, any data you enter will be reset periodically.",
     "your_projects": "Your Projects",
     "public": "Public",
     "private": "Private",

frontend/messages/ja.json

@@ -58,6 +58,7 @@
     "email_not_exist": "メールアドレスが見つかりません",
     "signup_error": "新規登録に失敗しました",
     "signin_error": "サインインに失敗しました",
+    "demo_page_warning": "これはデモ環境です。入力したデータは定期的にリセットされます。",
     "your_projects": "保有プロジェクト",
     "public": "パブリック",
     "private": "プライベート",