sidebar: right-align banner stack so 300px banners hug the column's right edge (matches content above/below)

src/components/SidebarAdStack.tsx

@@ -19,7 +19,7 @@ export default function SidebarAdStack({
   );
 
   return (
-    <div className={`space-y-4 ${className}`}>
+    <div className={`flex flex-col items-end gap-4 ${className}`}>
       {FIXED_300X600 && <AdImage ad={FIXED_300X600} priority />}
       {rotating.map((ad) => (
         <AdImage key={ad.src ?? ad.label} ad={ad} />