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/messages/de.json

@@ -175,6 +175,9 @@
 	"person_placeholder_notes": "Biographische Hinweise, Besonderheiten…",
 	"person_label_birth_year": "Geburtsjahr",
 	"person_label_death_year": "Todesjahr",
+	"person_label_generation": "Generation",
+	"person_option_generation_unset": "(keine)",
+	"person_hint_generation": "Generation in der Familie (G 0 = älteste Generation)",
 	"person_placeholder_year": "z.B. 1923",
 	"person_year_error": "Bitte eine vierstellige Jahreszahl eingeben",
 	"person_years_error_order": "Geburtsjahr muss vor dem Todesjahr liegen",
@@ -1103,7 +1106,6 @@
 	"stammbaum_relationships_heading": "Stammbaum & Beziehungen",
 	"stammbaum_zoom_in": "Vergrößern",
 	"stammbaum_zoom_out": "Verkleinern",
-	"stammbaum_generations": "Generationen",
 	"relation_error_duplicate": "Diese Beziehung gibt es bereits.",
 	"relation_error_circular": "Diese Beziehung würde einen Kreis erzeugen.",
 	"relation_error_self": "Eine Person kann nicht mit sich selbst verbunden werden.",