Feat: Add hierarchical category support for menu organization

Added parent-child relationship to categories, allowing organization
of categories into submenus (e.g., Apparel > Adults, Kids, Babies).

Changes:
- Add parentId field to Category model
- Create foreign key relationship for category hierarchy
- Update homepage to query only top-level categories (parentId = null)
- Include children categories for future use in category menus

Database migration:
- Adds parentId column to Category table
- Sets up CASCADE delete to remove children when parent is deleted

This enables menu structure like:
  Apparel
    - Adults
    - Kids
    - Babies
  Drinkware
  Phone Cases
  etc.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

prisma/migrations/20260721175000_add_parent_category_support/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable
+ALTER TABLE "Category" ADD COLUMN "parentId" TEXT;
+
+-- AddForeignKey
+ALTER TABLE "Category" ADD CONSTRAINT "Category_parentId_fkey" FOREIGN KEY ("parentId") REFERENCES "Category"("id") ON DELETE CASCADE ON UPDATE CASCADE;