Round the room preview dialog like everything around it (#34597)

The preview shown for a room you have not joined — the one carrying the
join and ask-to-join buttons — has 4px corners, where the room preview
card it stands in place of, the panel form of the same bar, and the
surfaces beside it all have 8px. Sitting among them it reads as sharp,
which is what was reported.

Nothing else in the rule changes, and the panel form already had the right
value, so the two forms of the bar now agree with each other as well.

Co-authored-by: David Baker <dbkr@users.noreply.github.com>

apps/web/res/css/views/rooms/_RoomPreviewBar.pcss

@@ -103,7 +103,8 @@ Please see LICENSE files in the repository root for full details.
     margin: auto;
     box-sizing: content;
     width: 400px;
-    border-radius: 4px;
+    /* Matches the room preview card this stands in place of, and the panel form of this same bar */
+    border-radius: 8px;
     flex-direction: column;
     padding: 20px;
     text-align: center;