fix(studio): use object-contain for thumbnails so portrait videos display correctly

object-cover was cropping 9:16 portrait videos to a narrow slice inside
the fixed landscape 80x45px thumbnail cells. object-contain shows the
full frame with neutral-background letterboxing. Applies to Renders tab,
Compositions tab, Assets tab, and timeline clip strip thumbnails.

packages/studio/src/components/sidebar/CompositionsTab.tsx

@@ -162,7 +162,7 @@ function CompCard({
           src={thumbnailUrl}
           alt={name}
           loading="lazy"
-          className="w-full h-full object-cover"
+          className="w-full h-full object-contain"
           onError={(e) => {
             (e.target as HTMLImageElement).style.display = "none";
           }}

packages/studio/src/components/ui/VideoFrameThumbnail.tsx

@@ -50,5 +50,5 @@ export function VideoFrameThumbnail({ src }: { src: string }) {
     return <div className="w-full h-full bg-neutral-800 animate-pulse" />;
   }
 
-  return <img src={frame} alt="" draggable={false} className="w-full h-full object-cover" />;
+  return <img src={frame} alt="" draggable={false} className="w-full h-full object-contain" />;
 }

packages/studio/src/player/components/CompositionThumbnail.tsx

@@ -112,7 +112,7 @@ export const CompositionThumbnail = memo(function CompositionThumbnail({
                 onLoad={(e) => {
                   (e.target as HTMLImageElement).style.opacity = "1";
                 }}
-                className="absolute inset-0 w-full h-full object-cover"
+                className="absolute inset-0 w-full h-full object-contain"
                 style={{ opacity: 0, transition: "opacity 200ms ease-out" }}
               />
             </div>