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/lib/generated/api.ts

@@ -1667,7 +1667,7 @@ export interface components {
             /** Format: int32 */
             deathYear?: number;
             /** Format: int32 */
-            generation?: number;
+            generation?: number | null;
         };
         Person: {
             /** Format: uuid */
@@ -1684,7 +1684,7 @@ export interface components {
             /** Format: int32 */
             deathYear?: number;
             /** Format: int32 */
-            generation?: number;
+            generation?: number | null;
             familyMember: boolean;
             sourceRef?: string;
             provisional: boolean;
@@ -2290,7 +2290,7 @@ export interface components {
             /** Format: int32 */
             deathYear?: number;
             /** Format: int32 */
-            generation?: number;
+            generation?: number | null;
             familyMember: boolean;
         };
         InferredRelationshipDTO: {