Implement most of the remaining layout changes

Includes the mobile UX optimizations and the tweaks we've made to cut down on wasted space, but does not yet include the change to embed the spotlight tile within the grid.

src/room/VideoPreview.module.css

@@ -18,20 +18,12 @@ limitations under the License.
   margin-inline: var(--inline-content-inset);
   min-block-size: 0;
   block-size: 50vh;
-}
-
-.preview.content {
-  margin-inline: 0;
-}
-
-.content {
+  border-radius: var(--cpd-space-4x);
   position: relative;
-  block-size: 100%;
-  inline-size: 100%;
   overflow: hidden;
 }
 
-.content video {
+.preview > video {
   width: 100%;
   height: 100%;
   object-fit: cover;
@@ -69,12 +61,20 @@ limitations under the License.
   );
 }
 
-.preview.content .buttonBar {
-  padding-inline: var(--inline-content-inset);
-}
-
 @media (min-aspect-ratio: 1 / 1) {
-  .preview video {
+  .preview > video {
     aspect-ratio: 16 / 9;
   }
 }
+
+@media (max-width: 550px) {
+  .preview {
+    margin-inline: 0;
+    border-radius: 0;
+    block-size: 100%;
+  }
+
+  .buttonBar {
+    padding-inline: var(--inline-content-inset);
+  }
+}