Settings: plain test notification + foreground banner delegate

- New 'Send plain test notification' button fires with zero criteria checks
  (no geofence, no cheapest check, no permission gate) to isolate delivery
  and tap behaviour
- ProximityMonitor is now the UNUserNotificationCenter delegate: banners
  show even in the foreground (system suppresses them otherwise), and taps
  complete the 'open FuelBoard' action

FuelBoard/SettingsView.swift

@@ -59,10 +59,15 @@ struct SettingsView: View {
                             .font(.footnote)
                             .foregroundStyle(.secondary)
                     }
+                    Button {
+                        ProximityMonitor.sendPlainTestNotification()
+                    } label: {
+                        Label("Send plain test notification", systemImage: "bell.slash.fill")
+                    }
                 } header: {
                     Text("Alerts")
                 } footer: {
-                    Text("Sends a notification exactly like a real price alert for \(alertsFuel.displayName.lowercased()) within \(distanceUnit.format(alertsRadiusKM)). Tapping it opens FuelBoard — just like a real alert.")
+                    Text("The first button sends a notification shaped like a real price alert for \(alertsFuel.displayName.lowercased()) within \(distanceUnit.format(alertsRadiusKM)). The second fires with no criteria at all — no geofence, no cheapest check — just to verify a notification appears and tapping it opens FuelBoard.")
                 }
 
                 Section {