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/components/PersonTypeSelector.svelte

@@ -2,9 +2,7 @@
 import { untrack } from 'svelte';
 import { radioGroupNav } from '$lib/actions/radioGroupNav';
 import { m } from '$lib/paraglide/messages.js';
-
-const TYPES = ['PERSON', 'INSTITUTION', 'GROUP', 'UNKNOWN'] as const;
-type PersonType = (typeof TYPES)[number];
+import { PERSON_TYPES as TYPES, type PersonType } from '$lib/person-validation';
 
 let {
 	value = 'PERSON',