Trends: chart key shows average price per station + descriptor footer

Legend rows now show each station's average over the days shown
in brackets (pence; signed +X.Xp in vs-cheapest mode, matching the
list-row deltas). Footer under the key explains the figure per mode.
averagePence helper (nil for empty series) + 3 tests; -showTrends
becomes a permanent QA hook like -showKeySheet.

FuelBoard/FavouritesView.swift

@@ -161,6 +161,13 @@ struct FavouritesView: View {
                     priceDisplayStyle: priceDisplayStyle
                 )
             }
+            .onAppear {
+                // QA hook: launch with `-showTrends` to open the sheet
+                // without a tap (same pattern as -showKeySheet).
+                if ProcessInfo.processInfo.arguments.contains("-showTrends") {
+                    showTrends = true
+                }
+            }
         }
     }
 }