Best Value: rename the remaining user-facing copy, hide the sheet section when off

Following the sort-segment rename, the rest of the user-facing "Worth the Drive"
copy moves to "Best Value".

- Settings: the toggle is now just "Best Value" (was "Enable Worth the Drive"),
  and the narrative footer opens "Best Value estimates fuel cost only, …".
- More info sheet: the section header is "Best Value" (was "Worth the drive?"),
  and the section itself is now conditional. `fuelSavings` is nil whenever the
  feature is off — `savingsLookup` returns nil when it is not configured — so
  the heading is never left dangling over an empty section while the feature is
  disabled.
- The unavailable text reads "Best Value unavailable: …".
- Comments and doc comments that named the sort were updated so they match the
  UI instead of contradicting it.

Deliberately unchanged: the enum's raw value stays `worthTheDrive`, so stored
sort preferences and widget configs keep decoding, and FuelSavingsStatus keeps
its verdict wording ("Worth the drive" / "Not worth the drive") — that describes
a station's verdict rather than the feature, and is not shown in the UI.

Verified on the simulator: with the feature OFF the more info sheet shows
Address / Distance / Directions and no heading at all; with it ON the sheet
shows the "Best Value" section; Settings reads "Fuel Savings" / "Best Value"
with no "Enable" anywhere. 172 tests, 0 failures.

FuelBoard/StationDetailView.swift

@@ -69,15 +69,21 @@ struct StationDetailView: View {
                             .background(Capsule().fill(Color.orange.opacity(0.14)))
                     }
                 }
-                Section {
-                    FuelSavingsSummaryView(
-                        availability: fuelSavings,
-                        amount: fuelSavingsAmount,
-                        baselineStationName: savingsBaselineStationName,
-                        distanceUnit: distanceUnit
-                    )
-                } header: {
-                    Text("Worth the drive?")
+                // Only shown while Fuel Savings backs it. `fuelSavings` is nil
+                // when the feature is off — `savingsLookup` returns nil when it
+                // is not configured — so the heading cannot be left dangling
+                // over an empty section.
+                if let fuelSavings {
+                    Section {
+                        FuelSavingsSummaryView(
+                            availability: fuelSavings,
+                            amount: fuelSavingsAmount,
+                            baselineStationName: savingsBaselineStationName,
+                            distanceUnit: distanceUnit
+                        )
+                    } header: {
+                        Text("Best Value")
+                    }
                 }
 
                 Section {