fix(post-detail): center carousel images of different sizes vertically

Regression from today's earlier "fill + object-contain" rework: fixing
that removed CarouselItem's flex items-center/justify-center, which is
what centered images in the carousel — because the shadcn Carousel
primitive's inner ref div (src/components/ui/carousel.tsx) never gets an
explicit height (its outer wrapper is hardcoded to just "overflow-hidden",
no className passthrough), the img's h-full never resolves to anything
definite there, so object-contain silently falls back to the image's
natural aspect-ratio sizing instead of fill+letterbox+center. Without
items-center, a shorter image among differently-sized carousel images
just sat flush at the top of the (taller, shared) row instead of centered.

The single-image path was unaffected — its container chain does resolve
to a definite height, so object-contain there works as intended.

src/components/feed/PostDetailDialog.tsx

@@ -267,7 +267,7 @@ export function PostDetailDialog({ post, open, onClose, onDeleted }: PostDetailD
         <Carousel setApi={setCarouselApi} className="h-full">
           <CarouselContent className="h-full">
             {images.map((img) => (
-              <CarouselItem key={img.id} className="basis-full h-full">
+              <CarouselItem key={img.id} className="basis-full h-full flex items-center justify-center">
                 <BlurImage
                   src={getMediaUrl(img.storageKey) ?? ""}
                   blurhash={img.blurhash}