Alerts choose their own fuel, independent of the Stations tab

The proximity monitor was tied to the Stations-tab fuel selection, so
switching the list to another fuel silently re-targeted geofences. Now the
Alerts tab has its own 'Fuel to monitor' picker (persisted as
fuelboard.alertsFuel):
- Monitor reads loadAlertsFuel() on init/background relaunch
- Changing the alerts fuel re-registers geofences around stations selling
  that fuel only
- Favourites priority slots stay scoped to the monitored fuel
- Tests: alerts fuel round-trip + default (33/33 passing)

FuelBoard/AlertsView.swift

@@ -1,10 +1,13 @@
 import SwiftUI
 
 /// Alerts tab — enables the cheapest-station proximity alerts and shows what's
-/// being monitored.
+/// being monitored. The fuel being monitored is chosen HERE, independently of
+/// the Stations-tab selection: users can browse any fuel without re-targeting
+/// their alerts (and vice versa).
 struct AlertsView: View {
     @Binding var enabled: Bool
     @Binding var radius: Double   // stored in km (monitor + storage)
+    @Binding var fuel: FuelType   // the fuel alerts monitor for
     let distanceUnit: DistanceUnit
     let monitoredCount: Int
     let lastAlert: String?
@@ -23,6 +26,19 @@ struct AlertsView: View {
                 }
 
                 if enabled {
+                    Section {
+                        Picker("Fuel", selection: $fuel) {
+                            ForEach(FuelType.allCases) { fuel in
+                                Text(fuel.shortName).tag(fuel)
+                            }
+                        }
+                        .pickerStyle(.segmented)
+                    } header: {
+                        Text("Fuel to monitor")
+                    } footer: {
+                        Text("Alerts watch for the cheapest \(fuel.displayName.lowercased()) station within the radius. Browsing a different fuel in the Stations tab does not change this.")
+                    }
+
                     Section("Trigger radius") {
                         VStack(alignment: .leading, spacing: 8) {
                             HStack {
@@ -47,10 +63,10 @@ struct AlertsView: View {
                                 .foregroundStyle(.secondary)
                         } else {
                             LabeledContent("Geofenced stations", value: "\(monitoredCount)")
-                            Text("Your favourites get priority, then the closest stations fill the rest (18 max, iOS region limit).")
+                            Text("Your favourites for \(fuel.displayName.lowercased()) get priority, then the closest stations selling that fuel fill the rest (18 max, iOS region limit).")
                                 .font(.caption)
                                 .foregroundStyle(.secondary)
-                            Text("Alerts are checked against the cheapest station within the trigger radius for the selected fuel. Each station alerts at most once per hour.")
+                            Text("Alerts are checked against the cheapest station within the trigger radius for the fuel chosen above. Each station alerts at most once per hour.")
                                 .font(.caption)
                                 .foregroundStyle(.secondary)
                         }