P1: Favourites Trends graph — FuelHistoryStore + TrendsView (Swift Charts)

- Shared/FuelHistory.swift: GitHub mirror read path (latest.json pointer +
  history/YYYY-MM-DD.json day files); day math (UTC noon keys), slim day decode
  with the shared price band, series + deltaSeries (vs-cheapest rebase),
  favourites-only app-group cache (90-day prune), parallel per-day fetches,
  404/network = graph gap never error.
- FuelPriceProvider: shared priceBand + mapGrades (single source for live +
  history decoding).
- FuelBoard/TrendsView.swift: line chart in the Favourites tab via toolbar
  button + sheet; fuel capsule (fuels with favourites only), 7/30/90 range,
  Price/vs-cheapest toggle, per-station legend, price-display style on y-axis,
  empty/loading/retry states with honest copy; no widget in v1.
- Localizable.strings: Trends keys.
- 83 tests (13 history + URL regression): day math, band guard, series gaps,
  delta rebasing, prune, and the appendingPathComponent regression (relative
  URL resolution dropped /main — all fetches 404'd).
- Sim-verified: favourites rows + TOP/deltas; sheet controls + building-up
  state with live first-snapshot date (archive has 1 day; lines render once
  >=2 snapshots).
- Backlog: P1 Trends DONE (unmerged); P0 REMAINING = live provider chain +
  telemetry beacon.

FuelBoard/FavouritesView.swift

@@ -30,6 +30,9 @@ struct FavouritesView: View {
     /// favourites (e.g. the last one is un-starred while viewing it).
     @State private var fuel: FuelType = .e10
 
+    /// Trends sheet (price history chart) presentation state.
+    @State private var showTrends = false
+
     private var activeFuel: FuelType {
         availableFuels.contains(fuel) ? fuel : (availableFuels.first ?? .e10)
     }
@@ -140,9 +143,24 @@ struct FavouritesView: View {
             .navigationTitle("Favourites")
             .toolbar {
                 if !favourites.isEmpty {
-                    EditButton()
+                    ToolbarItemGroup(placement: .topBarTrailing) {
+                        Button {
+                            showTrends = true
+                        } label: {
+                            Image(systemName: "chart.xyaxis.line")
+                                .accessibilityLabel("Trends")
+                        }
+                        EditButton()
+                    }
                 }
             }
+            .sheet(isPresented: $showTrends) {
+                TrendsView(
+                    favourites: favourites,
+                    selectedFuel: activeFuel,
+                    priceDisplayStyle: priceDisplayStyle
+                )
+            }
         }
     }
 }