Show the latest release and notify users about updates

## Problem
Users must manually check whether a newer Draft Omen version is available, and the website does not display the latest release version next to its download buttons.

## Proposed change
Display the latest available Draft Omen version beside the website download buttons. When the app launches, check for a newer release and, if one exists, show an app-styled notification identifying the available version with a button that opens the website download page. For example, version 0.3.0 should notify the user when 0.3.1 is available.

## Acceptance Criteria
- [ ] The website shows the latest available version next to its download buttons.
- [ ] The app checks for a newer release when it launches without blocking startup.
- [ ] No notification is shown when the installed version is current.
- [ ] When a newer version exists, an app-styled pop-up shows the installed and available versions.
- [ ] The pop-up includes a button that opens the website download page.
- [ ] Dismissing the pop-up allows the user to continue using the app normally.
- [ ] An unavailable update service or network failure does not interrupt app startup.

Start by locating the website download buttons and the app launch entry point. Trace how the latest release can be obtained without blocking startup, then implement the version display and update notification described in the acceptance criteria. Verify current, newer-version, dismissal, and unavailable-service cases.

索引モデルが issue の本文から書いたものです。