Make station price age informational only

The relay's price_updated is the maximum price_last_updated across the grades it maps, so it cannot vouch for the selected grade: stop using it to score, gate or warn about Best Value.

- FuelPriceFreshness is now station-level and display-only: one age per station, fresh/outdated/unknown/invalid, "May be outdated" past 30 days, with future-tolerance and non-finite guards.
- Best Value restores the closest-station-selling-the-selected-fuel baseline and its savings ranking; eligibility gating, eligible-first sorting and the unavailable banner are gone.
- Price age appears in the More info sheet only - no listing-row warnings, no extra banners.
- Copy reverted from "closest eligible station" to "closest station"; stale price-confidence strings removed, "May be outdated" added.
- Dormant per-grade timestamp plumbing removed, and regression tests added for every fuel type, missing/invalid/future stamps and the bundled offline dump.

FuelBoard/StationDetailView.swift

@@ -21,8 +21,6 @@ struct StationDetailView: View {
     var selectedFuel: FuelType = FuelStore.loadSelectedFuel()
     var fuelSavings: FuelSavingsAvailability? = nil
     var fuelSavingsAmount: FuelSavingsFillAmount = FuelSavingsSettings.defaultPreferredAmount
-    var savingsPriceConfidence: FuelPriceFreshness.ComparisonConfidence = .ineligible
-    var savingsPriceWarning: String? = nil
     var savingsBaselineStationName: String? = nil
     var journeyDiagnostics: FuelSavingsJourneyDiagnostics? = nil
     /// Whether this station is starred for the fuel currently being viewed, and
@@ -62,11 +60,11 @@ struct StationDetailView: View {
     }
 
     private var priceFreshness: FuelPriceFreshness.Assessment {
-        FuelPriceFreshness.assessment(for: station, fuel: selectedFuel)
+        FuelPriceFreshness.assessment(for: station)
     }
 
     /// The selected grade's current price, when present. Kept separate from
-    /// the freshness label so a stale price remains visible rather than being
+    /// the price-age row so an old price remains visible rather than being
     /// silently hidden.
     private var selectedFuelPriceText: String {
         guard let price = station.prices[selectedFuel] else { return "—" }
@@ -215,8 +213,7 @@ struct StationDetailView: View {
                     }
                     LabeledContent("\(selectedFuel.displayName) price", value: selectedFuelPriceText)
                         .monospacedDigit()
-                    LabeledContent("Price updated", value: priceFreshness.updateAgeLabel)
-                        .monospacedDigit()
+                    StationPriceAgeView(assessment: priceFreshness)
                     ForEach(StationNotice.notices(for: station)) { notice in
                         Label(notice.title, systemImage: notice.symbolName)
                             .font(.subheadline.weight(.semibold))
@@ -257,8 +254,6 @@ struct StationDetailView: View {
                         FuelSavingsSummaryView(
                             availability: fuelSavings,
                             amount: fuelSavingsAmount,
-                            priceConfidence: savingsPriceConfidence,
-                            priceWarning: savingsPriceWarning,
                             baselineStationName: savingsBaselineStationName,
                             distanceUnit: distanceUnit,
                             showInfoAction: { showBestValueInfo = true }