Fix vertical/portrait media getting cropped in single-attachment posts

[rufus-gh]

What & why

Closes #627 and #635.

A devlog/post with a single attachment rendered its media into a fixed 16:9 slide (aspect-ratio: 16 / 9; max-height: 420px) with object-fit: contain and overflow: hidden on the slide. For portrait images and vertical videos this:

• cropped the asset to its top slice only ([BUG] long video formats dont show up correctly #627, Devlog images aggressive cropping #635), and
• clipped the native video controls out of the box so they were unreachable ([BUG] long video formats dont show up correctly #627).

This matches @maxwofford's diagnosis on #627 that feed-post-card__video had an overflow cutoff it shouldn't have.

The fix

For the single-attachment case only ([data-count="1"]), let the media keep its own aspect ratio, height-capped at min(70vh, 560px), and set overflow: visible on the slide so video controls stay reachable. Multi-attachment grids are untouched (they still fill their square/16:9 slots).

&__media[data-count="1"] &__media-slide {
  aspect-ratio: auto;
  max-height: min(70vh, 560px);
  overflow: visible;
}

&__media[data-count="1"] &__image,
&__media[data-count="1"] &__video {
  height: auto;
  max-height: min(70vh, 560px);
}

Only app/assets/stylesheets/components/_feed.scss changes. The single render path is app/views/shared/_post_media.html.erb, used by both the feed and devlog detail pages, so the fix covers both surfaces.

Verification

Reproduced the crop and validated the fix in an isolated harness using the exact compiled rules, against a 9:16 portrait, a 16:9 landscape, and a 2-up grid:

• Portrait — before: only the top ~16:9 slice visible, controls clipped. After: full asset visible, controls reachable. ✅
• Landscape — visually identical before/after (no regression). ✅
• Multi-image grid — unchanged. ✅

Open questions

• min(70vh, 560px) is a judgement call for the height cap; happy to tune to whatever the team prefers.

[maxwofford]

@rufus-gh & @claude this doesn't follow our https://github.com/hackclub/stardance/blob/main/AI_POLICY.md- please include a screenshot of this working to prove that you tested it