fix(R-1218, R-1219): stage the off-site password when its file exists; no recovery ceremony during a recovery offer

R-1218: escrowStart stages when OffboxRepoPasswordOnBox() (not OffboxConfigured()). R-1219: the top bar hides during an
offer; the start handler answers 409 (api.escrow.recovery_offer_open, hu+en) before staging. Red-proved. Commit only.

controller/internal/backup/offbox.go

@@ -714,6 +714,14 @@ func (m *Manager) OffboxConfigured() bool {
 	return true
 }
 
+// OffboxRepoPasswordOnBox reports whether the off-site repository password FILE is on the box — whatever the target's
+// state. R-1218: ClearOffboxTarget keeps that file on purpose, and a switched-off target keeps it too, so a recovery
+// ceremony run then must still seal it; OffboxConfigured() is the wrong question for „is there a key to escrow".
+func (m *Manager) OffboxRepoPasswordOnBox() bool {
+	_, err := os.Stat(m.offboxPwPath())
+	return err == nil
+}
+
 // offboxRepoPwPattern matches a valid restic repo password (generateOffboxPassword = 32 rand bytes → 64 hex).
 var offboxRepoPwPattern = regexp.MustCompile(`^[0-9a-fA-F]{64}$`)