fix(core): re-layout ai-chat-content to display preview panel (#13030)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Introduced a split-view layout in the AI chat, allowing chat content
and a preview panel to be displayed side-by-side.
* Added responsive padding and layout adjustments for improved chat
panel appearance.

* **Refactor**
* Simplified the chat panel by removing the previous preview panel
feature and related state from the main chat component.
  * Updated internal logic to support the new split-view structure.

* **Style**
* Adjusted chat panel and workspace chat page styles for better layout
consistency and responsiveness.

* **Chores**
* Improved code organization and import statements for maintainability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/desktop/pages/workspace/chat/index.css.ts

@@ -3,9 +3,6 @@ import { style } from '@vanilla-extract/css';
 export const chatRoot = style({
   width: '100%',
   height: '100%',
-  maxWidth: 800,
-  padding: '0px 16px',
-  margin: '0 auto',
 });
 
 export const chatHeader = style({

packages/frontend/core/src/desktop/pages/workspace/chat/index.tsx

@@ -188,16 +188,18 @@ export const Component = () => {
 
     tool.session = currentSession;
 
+    tool.onNewSession = () => {
+      if (!currentSession) return;
+      setCurrentSession(null);
+      chatContent?.reset();
+    };
+
+    tool.onTogglePin = async () => {
+      await togglePin();
+    };
+
     // initial props
     if (!chatTool) {
-      tool.onNewSession = () => {
-        if (!currentSession) return;
-        setCurrentSession(null);
-        chatContent?.reset();
-      };
-      tool.onTogglePin = async () => {
-        await togglePin();
-      };
       // mount
       chatToolContainerRef.current.append(tool);
       setChatTool(tool);