client: add missing momentum-scroll hint to .cart-items

Fixes hyperhive-facing report (relayed via #66): scrolling broken on an
old iPad running an ancient WebKit build inside a kiosk browser
wrapper (WebFrame Pro 3.7.3).

.grid already carries -webkit-overflow-scrolling: touch (from #39,
the same class of old-WebKit scroll bug), but .cart-items - the other
touch-scrollable region, added in that same #39 follow-up - never got
it. On modern browsers this is a no-op; on old enough iOS WebKit,
touch gestures do not reliably scroll an overflow:auto container
without the explicit hint, which reads identically to a fully broken
scroll from the user's side.

Could not reproduce on the actual reported device (no access to it),
so flagging on the issue that this is the most concrete finding from
a code read, not a confirmed root-cause fix - checked for JS-level
touch/scroll blocking (none found) and viewport meta oddities
(user-scalable=no on index.html, standard for kiosk apps, not a
plausible cause on its own).

client/src/styles.css

@@ -228,6 +228,13 @@ button.danger { background: #5a2a2a; border-color: #7a3a3a; color: #fff; }
   flex: 1 1 0;
   min-height: 0;
   overflow-y: auto;
+  /* Old iOS WebKit needs the explicit momentum-scroll hint on every
+     touch-scrollable region, not just .grid (which already had it) — an
+     overflow: auto container works fine with mouse/trackpad without it,
+     but on an old-enough iOS build touch gestures don't reliably scroll
+     it at all, so from the user's side it reads identically to a broken
+     scroll rather than a missing animation. */
+  -webkit-overflow-scrolling: touch;
   margin-bottom: 6px;
 }
 .cart-items-inner {