fix(preview): drop double scrollbar in workspace

The outer <div className="relative flex-1 overflow-auto"> wrapping the
preview body was creating a second scroll track on top of whichever
scroll lives inside the body variant:

- mobile / tablet body already has its own overflow-auto
- desktop body is an iframe with intrinsic scrolling

The outer only needs to be a positioned parent for the absolutely
positioned TweakPanel, so switching to overflow-hidden removes the
redundant track without breaking Tweak drag bounds.

Author: hqhq1025

apps/desktop/src/renderer/src/components/PreviewPane.tsx

@@ -477,7 +477,7 @@ export function PreviewPane({ onPickStarter }: PreviewPaneProps) {
           </div>
         )}
         <CanvasErrorBar />
-        <div className="relative flex-1 overflow-auto">
+        <div className="relative flex-1 overflow-hidden">
           {body}
           {previewHtml ? <TweakPanel iframeRef={iframeRef} /> : null}
         </div>