Multi-theme compare route (litmus-yy2r)
Change compare from /compare/:left/:right to support 2-4 themes. N-column layout with horizontal scroll on mobile.
Summary of Changes
Changed compare route from /compare/:left/:right to /compare/:slugs with comma-separated slugs supporting 2-4 themes. N-column grid layout with compact scenes for 3+ themes. CompareSelector renders N dropdowns dynamically.