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>

src/app/page.tsx

@@ -40,8 +40,9 @@ export default async function HomePage() {
   const readyMadeCount = readyMade.length;
 
   const categoryRows = await prisma.category.findMany({
-    where: { showOnPersonalised: true },
+    where: { showOnPersonalised: true, parentId: null },
     orderBy: { name: 'asc' },
+    include: { children: { orderBy: { name: 'asc' } } },
   });
   const categories = categoryRows.map((c) => ({ label: c.name, value: c.slug }));