fix: post show layout matches editor and lightbox supports navigation

[paulocastellano]

Summary

• Mirror the post editor layout in the show page: media tiles on top in a 4-column grid (separate rounded cards, not joined inside a single card), caption below as plain text, labels at the bottom.
• Replace the custom <Teleport> lightbox with the shared ImagePreviewDialog component so multi-image posts get prev/next arrows, keyboard navigation, and click-outside-to-close.

Test plan

• Open a post with 4 images — verify the layout matches the editor (tiles on top, caption below).
• Click any image — verify the lightbox opens with arrows (only when there are 2+ items).
• Navigate with the arrows and with the keyboard (←/→).
• Click outside the image — verify the lightbox closes.
• Open a post with a single image — verify no arrows are shown.
• Open a post with a video — verify the video plays in the lightbox.
• Open a post with no media — verify the caption renders correctly with no spacing artifacts.