refactor(persons): centralise PersonType, PERSON_TYPES and normalizePersonType in person-validation

Removes four independent PersonType type declarations and the duplicated
TYPES/PERSON_TYPES arrays. normalizePersonType moves from the edit route
module into the shared lib so page.server.test.ts no longer imports from a
route. Both server actions now use normalizePersonType for personType
extraction instead of an inline type cast.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/person-validation.ts

@@ -1,3 +1,10 @@
+export const PERSON_TYPES = ['PERSON', 'INSTITUTION', 'GROUP', 'UNKNOWN'] as const;
+export type PersonType = (typeof PERSON_TYPES)[number];
+
+export function normalizePersonType(raw: string | undefined | null): PersonType {
+	return raw === 'SKIP' ? 'UNKNOWN' : ((raw ?? 'PERSON') as PersonType);
+}
+
 export function validatePersonFields(
 	personType: string,
 	firstName: string | undefined | null,