Add a MOTD feature

この issue には説明がありません。

The issue body is empty, so the first step is to understand what a MOTD (Message of the Day) feature entails for a game launcher. Look at the existing codebase for UI components and configuration loading, likely in src/ directory. Check how other features like game listing or settings are implemented to see where MOTD data could be fetched and displayed. Determine if it's a network call, a local file, or a static message, and where in the Tauri frontend (React) it should appear.

索引モデルが issue の本文から書いたものです。