fix: stronger drop shadow on heap card thumbnails

Bump the ActiveHeapCard stack thumbnails to a layered drop shadow and
a darker ring so they lift cleanly off the softened desert backdrop.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/heaps/ActiveHeapCard.tsx

@@ -121,7 +121,7 @@ export function ActiveHeapCard() {
                     marginLeft: -26, // half of w-13
                     marginTop: -26,  // half of h-13
                   }}
-                  className="h-[52px] w-[52px] rounded object-cover shadow-md ring-1 ring-black/40"
+                  className="h-[52px] w-[52px] rounded object-cover shadow-[0_4px_10px_rgba(0,0,0,0.65),0_2px_4px_rgba(0,0,0,0.5)] ring-1 ring-black/60"
                 />
               ))}
             </AnimatePresence>