Update the runtime to the latest version

This application relies on an outdated runtime that has been marked as end-of-life (EOL).

Software stores (GNOME Software, KDE Discovery, Bazaar, etc.) usually warn about EOL status and (depending on the software stores) discourage people from using EOL applications.

Please consider updating the runtime to the latest version.

Thank you.

```
LC_ALL=C flatpak remote-info flathub --system org.freedesktop.Platform//23.08
End-of-life: org.freedesktop.Platform 23.08 is no longer receiving fixes and security updates. Please update to a supported runtime version.
Date: 2025-09-29 15:36:17 +0000
```

No files, tests, or entry points are named in the issue. Start by reviewing linked pull request #22 and the project's Flatpak packaging configuration; done means the application uses a supported runtime and the reported EOL warning no longer appears.

索引モデルが issue の本文から書いたものです。