Sync the cart to an already-logged-in device, not just at login

Cross-device sync only ever worked one way (local→server on every
change via CartSync) plus a server→local merge at the exact moment of
login/register — a customer already signed in on a second device (no
fresh login action happening) never picked up changes made elsewhere.

Adds pullServerCart(): idempotent (server qty overwrites a matching
local line rather than adding to it, safe to call repeatedly, unlike
mergeServerCartIntoLocal()'s additive merge which only stays correct
right after a login clears the local cart's ambiguity). Triggered on
CartSync's mount and on window focus — covers "open the site while
already logged in" and "switch back to this tab after changing the
cart on another device," without a polling interval.

app/components/CartSync.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useEffect, useRef } from "react";
-import { useCart } from "../lib/cart";
+import { useCart, pullServerCart } from "../lib/cart";
 
 // Mirrors the local cart to the server whenever it changes, so a logged-in
 // customer's cart follows them across devices (see Customers.ts's `cart`
@@ -10,6 +10,14 @@ import { useCart } from "../lib/cart";
 // no-op when logged out — POST /api/account/cart 401s in that case, which
 // this component doesn't need to distinguish from success; there's simply
 // nothing to keep in sync yet.
+//
+// Push (local→server) and pull (server→local) are both handled here, but
+// deliberately asymmetric: push reacts to every local cart change (that's
+// this device's news to share), pull only runs on mount and on window
+// focus (see pullServerCart()'s own comment on why it's safe to call
+// repeatedly) — no polling interval, since "another device changed my
+// cart while this tab has been open and unfocused the whole time" is a
+// rare enough case not to justify a persistent timer.
 export function CartSync() {
   const cart = useCart();
   const isFirstRender = useRef(true);
@@ -34,5 +42,11 @@ export function CartSync() {
     return () => clearTimeout(timeout);
   }, [cart]);
 
+  useEffect(() => {
+    pullServerCart();
+    window.addEventListener("focus", pullServerCart);
+    return () => window.removeEventListener("focus", pullServerCart);
+  }, []);
+
   return null;
 }