Favourites: split list by fuel type with segmented tabs

Only fuel types that currently have at least one favourite appear as
tabs; each tab ranks that fuel's favourites cheapest-first and the
narrative (crown header, section title, cheapest-favourite explainer)
names the active fuel.

FuelBoard/FavouritesView.swift

@@ -1,21 +1,39 @@
 import SwiftUI
 
-/// Favourites tab — starred stations, ranked cheapest-first for the selected
-/// fuel, with the cheapest favourite called out at the top.
+/// Favourites tab — starred stations, split by fuel type. Each fuel type
+/// with at least one favourite appears as its own tab; that tab's favourites
+/// are ranked cheapest-first for that fuel, with the cheapest called out.
 struct FavouritesView: View {
     let favourites: [FuelStation]
+    /// The app's currently selected fuel — used only as the initial tab.
     let selectedFuel: FuelType
     let location: Coordinate?
     let distanceUnit: DistanceUnit
     let favouriteIDs: Set<String>
     var onToggleFavourite: (FuelStation) -> Void = { _ in }
 
-    /// Favourites that sell the selected fuel, cheapest first (distance tiebreak).
+    /// Fuel types that currently have at least one favourite selling them —
+    /// these are the only tabs shown.
+    private var availableFuels: [FuelType] {
+        FuelType.allCases.filter { fuel in
+            favourites.contains { $0.prices[fuel] != nil }
+        }
+    }
+
+    /// The active tab. Falls back if the chosen fuel ever loses all its
+    /// favourites (e.g. the last one is un-starred while viewing it).
+    @State private var fuel: FuelType = .e10
+
+    private var activeFuel: FuelType {
+        availableFuels.contains(fuel) ? fuel : (availableFuels.first ?? .e10)
+    }
+
+    /// Favourites that sell the active fuel, cheapest first (distance tiebreak).
     private var ranked: [FuelStation] {
-        let available = favourites.filter { $0.prices[selectedFuel] != nil }
+        let available = favourites.filter { $0.prices[activeFuel] != nil }
         return available.sorted { lhs, rhs in
-            let lPrice = lhs.prices[selectedFuel]!
-            let rPrice = rhs.prices[selectedFuel]!
+            let lPrice = lhs.prices[activeFuel]!
+            let rPrice = rhs.prices[activeFuel]!
             if lPrice != rPrice { return lPrice < rPrice }
             guard let location else { return false }
             return lhs.distanceKM(to: location.lat, lng2: location.lng) <
@@ -24,7 +42,22 @@ struct FavouritesView: View {
     }
 
     private var cheapest: FuelStation? { ranked.first }
-    private var cheapestPrice: Double? { cheapest?.prices[selectedFuel] }
+    private var cheapestPrice: Double? { cheapest?.prices[activeFuel] }
+
+    init(favourites: [FuelStation],
+         selectedFuel: FuelType,
+         location: Coordinate?,
+         distanceUnit: DistanceUnit,
+         favouriteIDs: Set<String>,
+         onToggleFavourite: @escaping (FuelStation) -> Void = { _ in }) {
+        self.favourites = favourites
+        self.selectedFuel = selectedFuel
+        self.location = location
+        self.distanceUnit = distanceUnit
+        self.favouriteIDs = favouriteIDs
+        self.onToggleFavourite = onToggleFavourite
+        _fuel = State(initialValue: selectedFuel)
+    }
 
     var body: some View {
         NavigationStack {
@@ -46,22 +79,31 @@ struct FavouritesView: View {
                         .padding(.vertical, 24)
                     }
                 } else {
+                    Section("Fuel type") {
+                        Picker("Fuel type", selection: $fuel) {
+                            ForEach(availableFuels) { fuel in
+                                Text(fuel.shortName).tag(fuel)
+                            }
+                        }
+                        .pickerStyle(.segmented)
+                    }
+
                     Section {
                         if let cheapest, let price = cheapestPrice {
                             HStack(spacing: 10) {
                                 Image(systemName: "crown.fill")
                                     .foregroundStyle(.yellow)
-                                Text("Cheapest favourite: \(cheapest.name) — \(String(format: "%.1fp", price))")
+                                Text("Cheapest \(activeFuel.displayName) favourite: \(cheapest.name) — \(String(format: "%.1fp", price))")
                                     .font(.footnote)
                             }
                         }
                     }
 
-                    Section("Favourites — cheapest first") {
+                    Section("\(activeFuel.displayName) favourites — cheapest first") {
                         ForEach(Array(ranked.enumerated()), id: \.element.id) { index, station in
                             StationRow(
                                 station: station,
-                                fuel: selectedFuel,
+                                fuel: activeFuel,
                                 location: location,
                                 distanceUnit: distanceUnit,
                                 baselinePrice: cheapestPrice,
@@ -74,7 +116,7 @@ struct FavouritesView: View {
 
                     if let cheapest = cheapest {
                         Section {
-                            Text("\(cheapest.name) is your cheapest favourite for \(selectedFuel.displayName). Favourites are monitored automatically — switch to the Alerts tab and enable alerts to get pinged when you approach one that's the cheapest within the radius.")
+                            Text("\(cheapest.name) is your cheapest \(activeFuel.displayName) favourite. Favourites are monitored automatically — switch to the Alerts tab and enable alerts to get pinged when you approach one that's the cheapest within the radius.")
                                 .font(.caption)
                                 .foregroundStyle(.secondary)
                         }