Settings: keep the Tap action description inside its card

The Tap action section rendered its description as a section footer, so the
text sat below the card while Units, Distances and Data checking show their
descriptions inside the card, separated from the control above by a hairline.
Move the same copy into the section body as a footnote/secondary row so the
card matches its neighbours. Wording is unchanged and no behavioural or
persistence code is touched.

FuelBoard/SettingsView.swift

@@ -263,10 +263,11 @@ struct SettingsView: View {
                     .onChange(of: stationTapAction) { _, newValue in
                         FuelStore.saveStationTapAction(newValue)
                     }
+                    Text("Choose what happens when you tap a station: get directions or view its details.")
+                        .font(.footnote)
+                        .foregroundStyle(.secondary)
                 } header: {
                     Text("Tap action")
-                } footer: {
-                    Text("Choose what happens when you tap a station: get directions or view its details.")
                 }
 
                 Section {