[Feature] Store categories and filter tags

[iiamlewis]

Overview

Shop needs categories and filter tags so players can browse by type (e.g. animals, memes, flags, fantasy, seasonal). Essential for discoverability as the catalogue grows.

Scope

• Category taxonomy (suggest: Animals, Memes, Flags, Fantasy, Sci-Fi, Seasonal, Exclusive)
• Filter tags on each product
• Category browse pages
• Filter by tag on main shop page
• Featured category slot

Acceptance criteria

• Each product has at least one category tag
• Players can browse by category
• Filter by multiple tags simultaneously
• Featured category displayed on shop homepage

Dependencies

• Depends on: Shop UI redesign