Add sub-categories to affirmation categories

Add self-referential parent/child relation on AffirmationCategory for
   one-level hierarchy. Update AffirmationForm with two-level category and
   sub-category dropdowns, server actions with find-or-create hierarchy
   logic, AffirmationList with Parent

prisma/schema.prisma

@@ -128,8 +128,11 @@ model SolfeggioFrequency {
 }
 
 model AffirmationCategory {
-  id           Int           @id @default(autoincrement())
-  name         String        @unique
+  id           Int                   @id @default(autoincrement())
+  name         String                @unique
+  parentId     Int?
+  parent       AffirmationCategory?  @relation("CategoryHierarchy", fields: [parentId], references: [id])
+  children     AffirmationCategory[] @relation("CategoryHierarchy")
   affirmations Affirmation[]
 }