[Feature] Store categories and filter tags

## 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