FIX: do not store 0 has min width for thead panel (#30818)

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.

plugins/chat/assets/javascripts/discourse/components/chat-side-panel.gjs

@@ -27,7 +27,7 @@ export default class ChatSidePanel extends Component {
     }
 
     const validWidth = Math.min(
-      this.store.getObject("width"),
+      this.store.getObject("width") ?? MIN_PANEL_WIDTH,
       this.mainContainerWidth - MIN_PANEL_WIDTH
     );
 

plugins/chat/assets/stylesheets/common/chat-navbar.scss

@@ -4,7 +4,6 @@
   background: var(--secondary);
   box-sizing: border-box;
   display: flex;
-  z-index: z("composer", "content") - 1;
   padding: 0 0.67em;
 
   &.-clickable {

plugins/chat/assets/stylesheets/common/chat-side-panel-resizer.scss

@@ -2,7 +2,7 @@
   top: 0;
   bottom: 0;
   position: absolute;
-  z-index: z("composer", "content") - 1;
+  z-index: 1;
   transition: background-color 0.15s 0.15s;
   background-color: transparent;
 

plugins/chat/assets/stylesheets/desktop/chat-side-panel.scss

@@ -9,7 +9,7 @@
   box-sizing: border-box;
   border-left: 1px solid var(--primary-low);
   position: relative;
-  min-width: 150px;
+  min-width: 250px;
 
   &__list {
     flex-grow: 1;