feat(webui): polish chat layout and titles

Align the WebUI sidebar and chat chrome with the updated design, and generate WebUI session titles asynchronously without blocking turns.

Co-authored-by: Cursor <cursoragent@cursor.com>

webui/src/components/thread/ThreadViewport.tsx

@@ -82,9 +82,9 @@ export function ThreadViewport({
             </div>
           </div>
         ) : (
-          <div className="mx-auto flex min-h-full w-full max-w-[64rem] flex-col px-4">
-            <div className="flex w-full flex-1 justify-center pb-16 pt-14 md:pt-[3.5rem]">
-              <div className="flex w-full max-w-[40rem] flex-col gap-5">
+          <div className="mx-auto flex min-h-full w-full max-w-[72rem] flex-col px-4">
+            <div className="flex w-full flex-1 items-center justify-center pb-[7vh] pt-8">
+              <div className="flex w-full max-w-[58rem] flex-col gap-6">
                 {emptyState}
                 <div className="w-full">{composer}</div>
               </div>