Simplify shared fill-up wording and help

FuelBoard/FuelSavingsViews.swift

@@ -4,6 +4,7 @@ struct FuelSavingsAdjustmentView: View {
     @Binding var amount: FuelSavingsFillAmount
 
     @State private var unitChangeNotice: String?
+    @State private var showingComparisonHelp = false
 
     private var range: ClosedRange<Double> {
         FuelSavingsCalculator.fillAmountSliderRange(for: amount.unit)
@@ -26,8 +27,33 @@ struct FuelSavingsAdjustmentView: View {
 
     var body: some View {
         VStack(alignment: .leading, spacing: 10) {
-            Text("How much are you filling up?")
-                .font(.subheadline.weight(.semibold))
+            HStack {
+                Text("How much fuel?")
+                    .font(.subheadline.weight(.semibold))
+                Spacer()
+                Button {
+                    showingComparisonHelp = true
+                } label: {
+                    Image(systemName: "info.circle")
+                }
+                .buttonStyle(.plain)
+                .foregroundStyle(.secondary)
+                .accessibilityLabel("About comparison modes")
+                .popover(isPresented: $showingComparisonHelp) {
+                    VStack(alignment: .leading, spacing: 12) {
+                        Text("Comparison modes")
+                            .font(.headline)
+                        Text("£: Compare how much fuel your budget buys at each station.")
+                            .fixedSize(horizontal: false, vertical: true)
+                        Text("Litres: Compare the cost of buying the same amount of fuel.")
+                            .fixedSize(horizontal: false, vertical: true)
+                    }
+                    .font(.footnote)
+                    .frame(width: 280, alignment: .leading)
+                    .padding()
+                    .presentationCompactAdaptation(.popover)
+                }
+            }
             Picker("Fill-up unit", selection: unitBinding) {
                 ForEach(FuelSavingsFillUnit.allCases) { unit in
                     Text(unit.displayName).tag(unit)
@@ -42,10 +68,7 @@ struct FuelSavingsAdjustmentView: View {
                     .monospacedDigit()
                     .frame(width: 72, alignment: .trailing)
             }
-            Text("Changes the estimate for this session only — your Settings default stays unchanged.")
-                .font(.caption2)
-                .foregroundStyle(.secondary)
-            Text("£ compares the litres your budget buys at each station; Litres compares one fixed quantity.")
+            Text("Adjust your fill-up amount for this visit. Your default won’t change.")
                 .font(.caption2)
                 .foregroundStyle(.secondary)
             if let unitChangeNotice {