Don't update counter if value hasn't changed

[andyra]

We're now lazy-loading cards, which means the #updateCounter method initially has 0 items to work with, and the --card-count variable will briefly be 0 before switching to the correct value once the cards load. That causes the CSS transition to fire on every Turbo navigation event.

We already have the correct value for --card-count set on the server, so the CSS will work like we want it to as long as the JS controller doesn't try to recalculate before the cards are loaded.

Edit: also see #2272