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/ContentView.swift

@@ -340,7 +340,7 @@ struct ContentView: View {
     private static var lastDriveLogLine: String?
 
     /// One summary line per ranking pass (FB-DRIVE) so a device console shows
-    /// whether the Worth the Drive list is actually carrying savings for the
+    /// whether the Best Value list is actually carrying savings for the
     /// whole pool — the regression was silent otherwise.
     private func logWorthTheDriveSummary(_ ordered: [FuelStation],
                                          baseline: FuelStation?,
@@ -1357,7 +1357,7 @@ struct StationRow: View {
             VStack(alignment: .trailing, spacing: 2) {
                 if isTopResult {
                     // The badge always marks the CHEAPEST PUMP PRICE (never
-                    // "best saving"), so in Worth the Drive order — where the
+                    // "best saving"), so in Best Value order — where the
                     // top row is the best net saving — it may sit lower down
                     // the list, and says so. It sits ABOVE the price it
                     // annotates rather than beside the name, so it reads as

FuelBoard/FuelSavingsViews.swift

@@ -142,7 +142,7 @@ struct FuelSavingsSummaryView: View {
                 }
             }
         case .unavailable(let reason):
-            Text(compact ? "Savings unavailable" : "Worth the Drive unavailable: \(reason)")
+            Text(compact ? "Savings unavailable" : "Best Value unavailable: \(reason)")
                 .font(compact ? .caption2 : .caption)
                 .foregroundStyle(.secondary)
         case nil:

FuelBoard/SettingsView.swift

@@ -150,13 +150,13 @@ struct SettingsView: View {
                 }
 
                 Section {
-                    Toggle("Enable Worth the Drive", isOn: fuelSavingsEnabledBinding)
+                    Toggle("Best Value", isOn: fuelSavingsEnabledBinding)
                     // The feature's own controls follow its toggle: switched
                     // off they grey out but stay on screen (same rule as the
                     // distance picker in Closest mode) and keep their values, so
                     // re-enabling restores exactly what was there. The sections
                     // that USE the feature — the in-list fill-up control and the
-                    // Worth the Drive sort option — are gated on `isConfigured`,
+                    // Best Value sort option — are gated on `isConfigured`,
                     // which already includes `enabled`, so they hide entirely.
                     Group {
                         HStack {
@@ -199,7 +199,7 @@ struct SettingsView: View {
                 } header: {
                     Text("Fuel Savings")
                 } footer: {
-                    Text("Worth the Drive estimates fuel cost only, using UK imperial MPG, your typical purchase amount, fuel prices and Apple Maps road distances. Real-world average MPG is better than manufacturer figures; the fill-up amount is your usual purchase, not tank capacity.")
+                    Text("Best Value estimates fuel cost only, using UK imperial MPG, your typical purchase amount, fuel prices and Apple Maps road distances. Real-world average MPG is better than manufacturer figures; the fill-up amount is your usual purchase, not tank capacity.")
                 }
 
                 Section {

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 {