Add the Key button to the Favourites tab

Favourites gains the same info button the Stations list has, last in
the trailing toolbar group so it sits to the right of Edit. It stays
visible when the list is empty, because the legend explains the prices
and labels before anything is starred — Trends and Edit still hide
with no favourites.

The legend itself is now one view, KeySheetView, lifted out of
StationsView's private computed property so both tabs present the
same sheet and cannot drift apart. It lives in StationsView.swift
because the key's row types are file-private there, which keeps the
Xcode project untouched. Stations passes its selected fuel's tint and
Favourites its active tab's, so the station-label pump badge matches
the rows on screen; everything else — detents, the two nested sheets,
and the -showKeySheet / -showBestValueInfo / -showImportantInfo QA
launch hooks — behaves exactly as before.

FuelBoard/FavouritesView.swift

@@ -48,6 +48,14 @@ struct FavouritesView: View {
     /// Trends sheet (price history chart) presentation state.
     @State private var showTrends = false
 
+    /// The Key (legend) sheet — the same sheet the Stations tab presents, opened
+    /// from the info button beside Edit.
+    @State private var showKey = false
+    /// The Key sheet's two nested sheets, owned here because its rows set them
+    /// (same arrangement as StationsView, so the QA hooks behave identically).
+    @State private var showBestValueInfo = false
+    @State private var showImportantInformation = false
+
     /// Station picked for the More-info sheet (tap action = showDetails).
     @State private var detailStation: FuelStation?
 
@@ -206,8 +214,8 @@ struct FavouritesView: View {
             }
             .navigationTitle(String(localized: "Favourites", comment: "Favourites tab title"))
             .toolbar {
-                if !favourites.isEmpty {
-                    ToolbarItemGroup(placement: .topBarTrailing) {
+                ToolbarItemGroup(placement: .topBarTrailing) {
+                    if !favourites.isEmpty {
                         Button {
                             showTrends = true
                         } label: {
@@ -216,8 +224,25 @@ struct FavouritesView: View {
                         }
                         EditButton()
                     }
+                    // Same button and same slot as the Stations tab: last in the
+                    // group, so it sits to the right of Edit. Shown even with no
+                    // favourites — the legend explains the prices and labels, which
+                    // matter before anything is starred.
+                    Button {
+                        showKey = true
+                    } label: {
+                        Image(systemName: "info.circle")
+                    }
+                    .accessibilityLabel(String(localized: "Key", comment: "Key button accessibility label"))
                 }
             }
+            .sheet(isPresented: $showKey) {
+                KeySheetView(
+                    fuelTint: activeFuel.tintColor,
+                    showBestValueInfo: $showBestValueInfo,
+                    showImportantInformation: $showImportantInformation
+                )
+            }
             .sheet(isPresented: $showTrends) {
                 TrendsView(
                     favourites: favourites,