fix(stammbaum): i18n the StammbaumCard heading (de/en/es)

Hardcoded 'Stammbaum & Beziehungen' heading replaced with
m.stammbaum_relationships_heading(); new key added to all
three message files.

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

frontend/messages/en.json

@@ -953,6 +953,7 @@
 	"stammbaum_panel_derived_rels": "Derived relationships",
 	"stammbaum_panel_to_person": "Go to person page →",
 	"stammbaum_panel_add_rel": "+ Add relationship",
+	"stammbaum_relationships_heading": "Family tree & relationships",
 	"stammbaum_zoom_in": "Zoom in",
 	"stammbaum_zoom_out": "Zoom out",
 	"stammbaum_generations": "Generations",