Prose text doesn't go full width

### What happened

What's up with the empty space, can we expand the prose, to go full width? Or is it by design?

### Version

0.0.9

### Platform

macOS

### Which editor's theme

_No response_

### Where deck looked

```text

```

### Did the window close on its own?

```text

```

No source file or test is named. Reproduce the layout on macOS with version 0.0.9, then trace the prose panel's width constraints in the desktop UI. Done means the prose uses the intended available width without harming the surrounding presentation layout, with the result verified against the reported screenshot.

Written by the indexing model from the issue text.