ui kit: preview picture scales to fit its stage

A clip or picture smaller than the preview used to stay at its own size inside black.
It now fills the stage and object-fit: contain scales it up; only a differing aspect ratio leaves bars.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E6LjCrZ8KMcHGZyRWjkPA5

app/ui/composer/elements/gallery.css

@@ -121,9 +121,11 @@
   min-block-size: 0; min-inline-size: 0;
   background: var(--void);
 }
+/* Fills the stage and lets object-fit do the fitting: with `auto` sizes and only a
+   max, a picture smaller than the stage stayed at its own size inside a field of
+   black instead of scaling up to it. */
 .cx-viewer-media {
-  max-inline-size: 100%; max-block-size: 100%;
-  inline-size: auto; block-size: auto;
+  inline-size: 100%; block-size: 100%;
   object-fit: contain;
 }