Key sheet: taller detent + opaque background

Sheet grows from .medium to .fraction(0.6) so the footer ('Colours
match each station's rating on the list') clears the bottom edge with
comfortable margin (verified: footer clearance +33pt). Sheet background
switched from the translucent default to a solid
systemGroupedBackground via .presentationBackground so the stations
list no longer shows through.

FuelBoard/StationsView.swift

@@ -255,7 +255,8 @@ struct StationsView: View {
                         }
                     }
                 }
-                .presentationDetents([.medium])
+                .presentationDetents([.fraction(0.6)])
+                .presentationBackground(Color(UIColor.systemGroupedBackground))
             }
         }
     }