Bundle reliability fixes

Two reliability improvements for Volume Bundles.

Free gifts always land in the cart. Products without an exposed primary variant (archived items, drafts, hidden-variant products) could slip into the gift picker. The picker now refuses them, and existing bundles repair themselves on the next save.

Add to Cart is double-fire safe. The bundle widget latches against re-entry, so one click never triggers more than one cart add — no matter how the theme wires the button.

Both fixes apply automatically on the next version push.