feat(stammbaum): bottom-right zoom + fit-to-screen control cluster (#692)

Move zoom controls out of the page header into a docked bottom-right cluster
inside the canvas (one-handed phone reach, Leonie) and add a fit-to-screen
button (data-testid=fit-to-screen). Add the 5 new i18n keys to de/en/es.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/messages/de.json

@@ -1106,6 +1106,11 @@
 	"stammbaum_relationships_heading": "Stammbaum & Beziehungen",
 	"stammbaum_zoom_in": "Vergrößern",
 	"stammbaum_zoom_out": "Verkleinern",
+	"stammbaum_fit_to_screen": "An Bildschirm anpassen",
+	"stammbaum_affordance_hint": "Ziehen zum Erkunden · Zusammendrücken zum Zoomen",
+	"stammbaum_affordance_dismiss": "Hinweis schließen",
+	"stammbaum_close_panel": "Schließen",
+	"stammbaum_centre_on_person": "Auf diese Person zentrieren",
 	"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.",

frontend/messages/en.json

@@ -1106,6 +1106,11 @@
 	"stammbaum_relationships_heading": "Family tree & relationships",
 	"stammbaum_zoom_in": "Zoom in",
 	"stammbaum_zoom_out": "Zoom out",
+	"stammbaum_fit_to_screen": "Fit to screen",
+	"stammbaum_affordance_hint": "Drag to explore · pinch to zoom",
+	"stammbaum_affordance_dismiss": "Dismiss hint",
+	"stammbaum_close_panel": "Close",
+	"stammbaum_centre_on_person": "Centre on this person",
 	"relation_error_duplicate": "This relationship already exists.",
 	"relation_error_circular": "This relationship would form a cycle.",
 	"relation_error_self": "A person cannot be related to themselves.",

frontend/messages/es.json

@@ -1106,6 +1106,11 @@
 	"stammbaum_relationships_heading": "Árbol genealógico & relaciones",
 	"stammbaum_zoom_in": "Acercar",
 	"stammbaum_zoom_out": "Alejar",
+	"stammbaum_fit_to_screen": "Ajustar a la pantalla",
+	"stammbaum_affordance_hint": "Arrastra para explorar · pellizca para ampliar",
+	"stammbaum_affordance_dismiss": "Cerrar aviso",
+	"stammbaum_close_panel": "Cerrar",
+	"stammbaum_centre_on_person": "Centrar en esta persona",
 	"relation_error_duplicate": "Esta relación ya existe.",
 	"relation_error_circular": "Esta relación crearía un ciclo.",
 	"relation_error_self": "Una persona no puede estar relacionada consigo misma.",