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/utils/gifPreviewSize.ts

@@ -0,0 +1,39 @@
+/**
+ * Display size for a Giphy / Tenor link-preview GIF. The card used to force
+ * every GIF into a full-width × 200px band with `object-fit: cover`, so square
+ * and portrait GIFs lost most of their height and small ones were blown up.
+ * Fit the GIF inside `maxWidth × maxHeight` keeping its aspect ratio, and
+ * enlarge a small GIF at most `maxUpscale`× so it stays sharp.
+ */
+
+/** The GIF card's width (UrlPreview is 25rem); a phone shrinks it further via CSS. */
+export const GIF_PREVIEW_MAX_WIDTH = 400;
+export const GIF_PREVIEW_MAX_HEIGHT = 320;
+export const GIF_PREVIEW_MAX_UPSCALE = 2;
+
+export type GifPreviewBox = { width: number; height: number };
+
+// og:image:width / og:image:height are numbers from Synapse, but some sites
+// publish them as strings; anything else (missing, 0, NaN) means "unknown".
+const dimension = (value: unknown): number | undefined => {
+  const n = typeof value === 'string' ? Number(value) : value;
+  return typeof n === 'number' && Number.isFinite(n) && n > 0 ? n : undefined;
+};
+
+/** The box to render the GIF in, or `undefined` when its size isn't known. */
+export const gifPreviewBox = (
+  rawWidth: unknown,
+  rawHeight: unknown,
+  maxWidth = GIF_PREVIEW_MAX_WIDTH,
+  maxHeight = GIF_PREVIEW_MAX_HEIGHT,
+  maxUpscale = GIF_PREVIEW_MAX_UPSCALE,
+): GifPreviewBox | undefined => {
+  const width = dimension(rawWidth);
+  const height = dimension(rawHeight);
+  if (!width || !height) return undefined;
+  const scale = Math.min(maxWidth / width, maxHeight / height, maxUpscale);
+  return {
+    width: Math.max(1, Math.round(width * scale)),
+    height: Math.max(1, Math.round(height * scale)),
+  };
+};