fix: handle invalid GIF dimensions gracefully

Fix NaN aspectRatio when rehydrating GIFs from drafts by:

1. Adding validation in parseTenorGif to reject invalid dimensions
   (NaN, zero, or negative values)
2. Adding defensive checks in GifEmbed to fallback to 1:1 aspect
   ratio if dimensions are invalid
3. Adding defensive checks in composer ExternalEmbedGif to fallback
   to 16:9 if gif.media_formats.gif.dims is missing or invalid

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/view/com/composer/ExternalEmbed.tsx

@@ -37,7 +37,13 @@ export const ExternalEmbedGif = ({
   )
 
   const loadingStyle: ViewStyle = {
-    aspectRatio: gif.media_formats.gif.dims[0] / gif.media_formats.gif.dims[1],
+    aspectRatio: (() => {
+      const dims = gif.media_formats.gif?.dims
+      if (dims && dims[0] > 0 && dims[1] > 0) {
+        return dims[0] / dims[1]
+      }
+      return 16 / 9 // Default aspect ratio
+    })(),
     width: '100%',
   }