Thread panel width is forgotten after restarting the desktop app

### Describe the bug

Resizing the thread side panel does not stick. After quitting and reopening Buzz Desktop, the panel is back at its default width and has to be dragged out again every time.

Related: PR #4817 addresses this ("persist thread panel width across restarts"), open and unreviewed since 2026-08-05. No existing issue found.

### Steps to reproduce

1. Open a channel and open any thread so the thread panel appears next to it.
2. Drag the divider to make the panel noticeably wider.
3. Quit Buzz Desktop completely and start it again.
4. Open a thread in the same channel.

### Expected behavior

The panel opens at the width it was last dragged to, and stays there across restarts.

### Version and platform

Buzz Desktop v0.5.23, macOS.

### Logs / screenshots

n/a