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/migrations/20260223163626_add_category_parent/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable
+ALTER TABLE "AffirmationCategory" ADD COLUMN     "parentId" INTEGER;
+
+-- AddForeignKey
+ALTER TABLE "AffirmationCategory" ADD CONSTRAINT "AffirmationCategory_parentId_fkey" FOREIGN KEY ("parentId") REFERENCES "AffirmationCategory"("id") ON DELETE SET NULL ON UPDATE CASCADE;