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

src/lib/components/admin/AdminTable.svelte

@@ -110,7 +110,7 @@
 								class="flex items-center gap-1 hover:text-gray-900"
 								onclick={() => toggleSort(field)}
 							>
-								{fieldNameToLabel(field.includes('.') ? (field.split('.').pop() ?? field) : field)}
+								{fieldNameToLabel(field.includes('.') ? field.split('.')[0] : field)}
 								<span class="text-xs text-gray-400">{getSortIndicator(field)}</span>
 							</button>
 						</th>