New app icon (drip: £ fuel drop, green border, gradient bg) + Alerts radius guidance labels

- AppIcon / AppIcon-dark / AppIcon-tinted swapped for the drip design
  (1024px, dark + tinted variants), onboarding artwork matched
- Alerts tab: live explainer under the Radius picker — situational
  guidance per option (Follow search = set-and-forget mirror of the
  Stations distance; 1 = town driving; 2 = city/suburb commute;
  3 = mixed; 5 = A-roads/motorway exits; 8 = long-distance) — plus the
  trade-off line in the section footer
- 83 tests, Release build green

FuelBoard/AlertsView.swift

@@ -46,6 +46,34 @@ struct AlertsView: View {
         return "\(shown) \(distanceUnit.label(for: Double(shown)))"
     }
 
+    /// Situational guidance for the currently-selected alert radius, shown
+    /// under the picker so users know what each choice is best for. Labels
+    /// mirror the picker row text (unit-aware) and explain the trade-off:
+    /// smaller radii ping more often but later; larger radii ping rarely in
+    /// town but earlier from afar.
+    private var alertRadiusGuidance: String {
+        switch alertChoice.wrappedValue {
+        case .followSearch:
+            return "Set-and-forget: alerts behave exactly like the Stations-tab search distance (capped at 8 miles for reliable geofencing)."
+        case .fixed(let value):
+            let optionLabel = "\(value) \(distanceUnit.label(for: Double(value)))"
+            let body: String
+            switch value {
+            case 1:
+                body = "Town driving. Fires most often — you're rarely inside the circle for long, so pings are meaningful. Best for stop-start local trips."
+            case 2:
+                body = "City or suburb commute. Slightly earlier heads-up than the smallest option; still fires regularly around town."
+            case 3:
+                body = "Mixed driving. Middle ground: heads-up a few minutes out, still works in most towns."
+            case 5:
+                body = "A-roads and motorway exits. Wide early warning for longer trips — but in town it rarely fires, because you're usually already inside the circle."
+            default:
+                body = "Long-distance journeys. Maximum reach: a ping when a cheap station is coming up from far out. In town this essentially never fires."
+            }
+            return "\(optionLabel) — \(body)"
+        }
+    }
+
     private enum AlertRadiusChoice: Hashable {
         case followSearch
         case fixed(Int)
@@ -115,6 +143,9 @@ struct AlertsView: View {
                             Text("\(value) \(distanceUnit.label(for: Double(value)))").tag(AlertRadiusChoice.fixed(value))
                         }
                     }
+                    Text(alertRadiusGuidance)
+                        .font(.caption)
+                        .foregroundStyle(.secondary)
                     if enabled && monitoredCount == 0 {
                         Text("No stations monitored yet — open the Stations tab to load prices first.")
                             .font(.caption)
@@ -123,7 +154,7 @@ struct AlertsView: View {
                 } header: {
                     Text("Cheapest-station alerts")
                 } footer: {
-                    Text("When you approach a station that is the cheapest within the radius, FuelBoard sends a notification — even with the app closed. Nearby stations selling \(fuel.displayName.lowercased()) are watched — favourites get priority — and each station alerts at most once per hour. Follow search mirrors the Stations-tab distance, capped at 8 miles for reliable geofencing.")
+                    Text("When you approach a station that is the cheapest within the radius, FuelBoard sends a notification — even with the app closed. Nearby stations selling \(fuel.displayName.lowercased()) are watched — favourites get priority — and each station alerts at most once per hour. Follow search mirrors the Stations-tab distance, capped at 8 miles for reliable geofencing. Smaller radii ping more often but later; larger radii ping rarely in town but earlier from afar.")
                 }
 
                 if enabled, let lastAlert {