MM-59038 Fix markdown inconsistencies in preview mode (#30719)

* fix inconsistency with markdown image sizes in preview mode

* fix preview mode in RHS so it shows HR

* Update _markdown.scss

* fix css linting issue

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/sass/components/_post-right.scss

@@ -167,6 +167,12 @@
         border: none;
     }
 
+    .textarea-wrapper-preview {
+        hr {
+            border-top: solid 1px rgba(var(--center-channel-color-rgb), 0.24)
+        }
+    }
+
     .post-create__container {
         width: 100%;
 

webapp/channels/src/sass/layout/_markdown.scss

@@ -51,7 +51,8 @@ h6.markdown__heading {
 }
 
 #post-list,
-#rhsContainer {
+#rhsContainer,
+.textbox-preview-area {
     .markdown-inline-img {
         max-height: 500px;
         border-radius: 4px;
@@ -147,6 +148,16 @@ h6.markdown__heading {
     }
 }
 
+.textbox-preview-area {
+    .markdown-inline-img--hover {
+        &:hover {
+            border: 1px solid transparent;
+            box-shadow: none;
+            cursor: default;
+        }
+    }
+}
+
 .post-code {
     position: relative;
     overflow-x: auto;