💄(frontend) add symmetric shadows for white button contrast enhancement

Add symmetric shadows at top and bottom of white circle buttons to
ensure sufficient visual contrast against varying background colors.

Improves button visibility and accessibility by providing consistent
visual definition regardless of background context.

src/frontend/src/features/rooms/components/Join.tsx

@@ -410,6 +410,28 @@ export const Join = ({
                 alignItems: 'center',
               })}
             >
+              <div
+                className={css({
+                  position: 'absolute',
+                  top: 0,
+                  height: '5rem',
+                  width: '100%',
+                  backgroundImage:
+                    'linear-gradient(to bottom, rgba(0, 0, 0, 0.6) 0%, rgba(0, 0, 0, 0.3) 40%, rgba(0, 0, 0, 0.1) 80%, rgba(0, 0, 0, 0) 100%)',
+                  zIndex: 1,
+                })}
+              />
+              <div
+                className={css({
+                  position: 'absolute',
+                  bottom: 0,
+                  height: '5rem',
+                  width: '100%',
+                  backgroundImage:
+                    'linear-gradient(to top, rgba(0, 0, 0, 0.6) 0%, rgba(0, 0, 0, 0.3) 35%, rgba(0, 0, 0, 0.1) 75%, rgba(0, 0, 0, 0) 100%)',
+                  zIndex: 1,
+                })}
+              />
               <div
                 className={css({
                   position: 'relative',