[US-020] Browse stores by category

[leimo2011]

Epic: EP-04 — End User Discovery \nPriority: Must Have (M) \nStory Points: 5 \nSprint: Sprint 2\n\n### User Story\nAs an end user, I want to browse stores by category so I can discover sellers in my area of interest.\n\n### Acceptance Criteria\n- [ ] Category grid/list shown on home screen\n- [ ] Tapping a category shows paginated list of stores in that category\n- [ ] Store cards show logo, name, live indicator, avg rating\n- [ ] Categories configurable by super admin\n- [ ] Empty state shown for categories with no active stores\n\n### Technical Notes\n- GET /api/categories/ — list active categories\n- GET /api/stores/?category={slug} — filtered stores