When we had no width stored for the side panel in the local storage,
essentially the computation would end up being:
```javascript
Math.min(null, 1000);
```
Which would output: 0. This commit ensures we have a default for store
width: MIN_PANEL_WIDTH. And also uses the same value in CSS and JS.
I had to change z-layers of chat-side-panel resizer as it was
interfering with system specs changing the notification level of a
thread, when trying to click the option in the dropdown, it would
instead click the resizer as it would appear on top of the dropdown.
Tried to write a test but couldn't get something reliable.
6c3a7f2a67