fix(embeds): GIF previews keep their own size and shape (#341)

Giphy and Tenor link previews forced every GIF into the same box — full
card width, max 200px tall, object-fit: cover — whatever its shape: a
square GIF lost half its height, a portrait one three quarters (only the
middle band showed, so captions and faces at the top/bottom were cut), and
a 100×80 GIF was blown up 4×.

utils/gifPreviewSize.ts (unit-tested) fits the GIF inside 400×320 from
og:image:width/height, keeping its aspect ratio and enlarging small GIFs
at most 2×. The card centres the GIF on the surface colour with
object-fit: contain, reserves the box before it loads, and the GIF badge
sits on the GIF's corner. Without dimensions it shows at natural size,
capped by the card. The >10 MB thumbnail fallback is requested at 2× the
box (800×640) so it isn't upscaled.

Verified headless (fixture previews, desktop / Pixel 7 / 320px phone):
480×200 → 398×166, 300×300 → 320×320 (292×292 on 320px), 240×480 →
160×320, 100×80 → 200×160, all 100% visible; the no-dimension fallback
and narrow cards keep the ratio. tsc clean, 1325 unit tests pass, eslint
0 errors (36 warnings, unchanged), prettier clean.

Fixes #341

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/url-preview/UrlPreview.css.tsx

@@ -1,5 +1,6 @@
 import { keyframes, style } from '@vanilla-extract/css';
 import { DefaultReset, color, config, toRem } from 'folds';
+import { GIF_PREVIEW_MAX_HEIGHT } from '../../utils/gifPreviewSize';
 
 export const UrlPreview = style([
   DefaultReset,
@@ -955,12 +956,14 @@ export const PortraitSideLayout = style([
 // GIF card (Giphy / Tenor)
 // ---------------------------------------------------------------------------
 
+// Full card width; a GIF narrower than the card (portrait, square, small) is
+// centred on the surface colour instead of being stretched and cropped.
 export const GifThumbnailWrapper = style([
   DefaultReset,
   {
-    position: 'relative',
+    display: 'flex',
+    justifyContent: 'center',
     width: '100%',
-    maxHeight: toRem(200),
     overflow: 'hidden',
     flexShrink: 0,
     backgroundColor: color.Surface.Container,
@@ -972,14 +975,26 @@ export const GifThumbnailWrapper = style([
   },
 ]);
 
+// Wraps just the GIF so the "GIF" badge sits on the GIF's own corner.
+export const GifFrame = style([
+  DefaultReset,
+  {
+    position: 'relative',
+    display: 'block',
+    maxWidth: '100%',
+  },
+]);
+
+// The inline width/aspect-ratio (from og:image:width/height) sets the size;
+// max-width lets a phone's narrower card shrink it, keeping the ratio.
 export const GifThumbnailImg = style([
   DefaultReset,
   {
-    width: '100%',
-    maxHeight: toRem(200),
-    objectFit: 'cover',
-    objectPosition: 'center',
     display: 'block',
+    maxWidth: '100%',
+    maxHeight: toRem(GIF_PREVIEW_MAX_HEIGHT),
+    height: 'auto',
+    objectFit: 'contain',
   },
 ]);