Alerts tab: picker-style config matching Live Activity; move Live Activity there

- Alerts section restyled to mirror the Live Activity section: one section
  with toggle + Fuel picker + Radius picker (was segmented fuel + slider)
- Alert radius options expanded to 1/2/3/5/8/10/15/20 (display unit, snapped
  to nearest option; stored km unchanged; validation widened to 33 km)
- Live Activity section moved from Settings into the Alerts tab so both
  'notify while driving' surfaces sit together
- SettingsView loses the Live Activity bindings/section

FuelBoard/ContentView.swift

@@ -159,6 +159,9 @@ struct ContentView: View {
                 enabled: $alertsEnabled,
                 radius: $alertsRadius,
                 fuel: $alertsFuel,
+                liveActivityEnabled: $liveActivityEnabled,
+                liveActivityFuel: $liveActivityFuel,
+                liveActivityRadiusMiles: $liveActivityRadiusMiles,
                 distanceUnit: distanceUnit,
                 monitoredCount: monitor.monitoredStationIDs.count,
                 lastAlert: monitor.lastAlert
@@ -325,9 +328,6 @@ struct ContentView: View {
     private var settingsTab: some View {
         SettingsView(
             distanceUnit: $distanceUnit,
-            liveActivityEnabled: $liveActivityEnabled,
-            liveActivityFuel: $liveActivityFuel,
-            liveActivityRadiusMiles: $liveActivityRadiusMiles,
             alertsFuel: alertsFuel,
             alertsRadiusKM: alertsRadius,
             testAlertResult: monitor.lastTestResult,