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.
Marco
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