feat(person): generation dropdown on Person edit/new forms (#689)

PersonEditForm.svelte gains a G 0…G 6 select inside the {#if isPerson}
block. min-h-[44px] meets WCAG 2.5.8 / dual-audience touch target.
generationStr is initialised via $state(untrack(...)) so prop reruns
never reset an in-progress edit (same pattern as selectedType).

Both /persons/[id]/edit and /persons/new form actions read the field
without the conditional-spread idiom — generation always lands in the
PUT/POST body. G 0 is a valid family-tree-root value the spread would
silently drop, and an empty option sends null so a human can clear the
field back to "unset".

i18n adds person_label_generation / person_option_generation_unset /
person_hint_generation in de/en/es. Drops the dead stammbaum_generations
key (zero callsites after the filter-chip removal in the spec).

Tests: dropdown render + hydration in the component, generation=0/3/null
arriving in the API body in the server actions.

Refs #689

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/routes/persons/[id]/edit/+page.server.ts

@@ -51,6 +51,12 @@ export const actions = {
 		const deathYearStr = formData.get('deathYear')?.toString().trim();
 		const birthYear = birthYearStr ? parseInt(birthYearStr, 10) : undefined;
 		const deathYear = deathYearStr ? parseInt(deathYearStr, 10) : undefined;
+		// Must NOT use the conditional-spread idiom for generation: G 0 is a
+		// valid family-tree-root value. The key always travels in the body so
+		// an explicit clear (empty option) reaches the backend as null.
+		const generationRaw = formData.get('generation');
+		const generation =
+			generationRaw == null || generationRaw.toString() === '' ? null : Number(generationRaw);
 
 		const validationKey = validatePersonFields(personType, firstName, lastName);
 		if (validationKey) {
@@ -68,7 +74,8 @@ export const actions = {
 				...(alias ? { alias } : {}),
 				...(notes ? { notes } : {}),
 				...(birthYear ? { birthYear } : {}),
-				...(deathYear ? { deathYear } : {})
+				...(deathYear ? { deathYear } : {}),
+				generation
 			}
 		});