UX: add HTML class for composer preview to manage sidebar height (#30956)

This adds the class `composer-has-preview` to the HTML element when the
composer is opened and the preview is visible.

This allows us to adjust the sidebar height, so that the previewless
composer can overlap the sidebar and better utilize the available space.


Before:

![image](https://github.com/user-attachments/assets/8780f159-5d24-4ac0-8a4f-43a9b1665532)



After: 

![image](https://github.com/user-attachments/assets/baf69856-0ece-453f-aa0f-e7be9939af97)

app/assets/javascripts/discourse/app/components/composer-container.hbs

@@ -8,6 +8,7 @@
 >
   <div class="grippie"></div>
   {{#if this.composer.visible}}
+    {{html-class (if this.composer.showPreview "composer-has-preview")}}
     <ComposerMessages
       @composer={{this.composer.model}}
       @messageCount={{this.composer.messageCount}}

app/assets/stylesheets/common/base/sidebar.scss

@@ -88,6 +88,10 @@
 
     // allows sidebar to scroll to the bottom when the composer is open
     height: calc(100% - var(--composer-height, 0px));
+
+    html:not(.composer-has-preview) & {
+      height: 100%;
+    }
   }
 
   .sidebar-sections {