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_

Start by reviewing the Settings > Themes / Appearance area and the desktop platforms’ native compositor capabilities. Done means users can adjust window opacity and optionally enable a blur-behind effect, with the behavior supported across the relevant desktop environments.

Written by the indexing model from the issue text.