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/schema.prisma

@@ -10,10 +10,14 @@ datasource db {
 // Categories are their own table so you can add/rename/remove them from the admin
 // panel. Product.category stays a plain string (matching Category.slug) rather
 // than a foreign key, so nothing breaks if a category is renamed after the fact.
+// Categories can be hierarchical: parentId links to a parent category.
 model Category {
   id        String   @id @default(cuid())
   name      String // display label, e.g. "Phone Cases"
   slug      String   @unique // stored key, e.g. "PHONE_CASE" — matches Product.category
+  parentId  String? // parent category ID for hierarchical organization (e.g., "Apparel - Adults" has parent "Apparel")
+  parent    Category? @relation("CategoryHierarchy", fields: [parentId], references: [id], onDelete: Cascade)
+  children  Category[] @relation("CategoryHierarchy")
   showOnPersonalised Boolean @default(true) // appears as a filter on /made-to-order
   showOnProducts     Boolean @default(true) // appears as a filter on /products
   createdAt DateTime @default(now())