Window background transparency / opacity control

### Feature summary

Add support for adjusting the window background opacity or enabling a transparency/blur effect, similar to what other modern desktop apps offer.

### What problem are you trying to solve?

Transparent or translucent windows let users keep visual context of content behind the app — documentation, terminals, reference material — while working. This is a popular customization in developer tooling and would improve the overall desktop experience.

### Proposed solution

A new option under Settings > Themes / Appearance to:

- Adjust window opacity — a slider or numeric input to set how transparent the window background is
- Enable blur-behind — an optional frosted glass / translucent effect using the OS native compositor

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

まず Settings > Themes / Appearance エリアと、デスクトッププラットフォームのネイティブなコンポジター機能を確認します。ユーザーがウィンドウの不透明度を調整でき、必要に応じて背後のぼかし効果を有効にでき、関連するデスクトップ環境全体でその動作がサポートされれば完了です。

索引モデルが issue の本文から書いたものです。