Use SDC for product teasers

Problem/Motivation

Product teaser variants (Belgrade, Card and Zoom) use the legacy architecture with files scattered across scss/components/product-teaser/, css/components/product-teaser/, templates/commerce/teasers/, and belgrade.libraries.yml, requiring manual library attachments and multiple file locations for maintenance.

Migrate both teaser variants to self-contained SDC components in components/product-teaser-belgrade/ and components/product-teaser-zoom/ following the existing content-pane pattern, then update the commerce product teaser templates to use {% include 'belgrade:product-teaser-*' %} instead of manual library loading.

We could also consider adding new product teasers and the card teaser may require some work.