Add first-launch onboarding with location + notification permission prompts

- OnboardingView: 4-page flow (welcome + features, location, notifications, done)
- Triggers system prompts in-context via OnboardingPermissionPrompter
- Shown at initial launch only (FuelStore.onboardingCompleted flag, app-group defaults)
- Test hook: 'Show onboarding (testing)' button at bottom of Alerts tab

FuelBoard/ContentView.swift

@@ -18,6 +18,7 @@ struct ContentView: View {
     }()
     @State private var isLoading = false
     @State private var statusMessage = ""
+    @State private var showOnboarding = false
     @State private var locationManager = LocationManager()
     @StateObject private var monitor = ProximityMonitor()
 
@@ -132,18 +133,37 @@ struct ContentView: View {
                 enabled: $alertsEnabled,
                 radius: $alertsRadius,
                 monitoredCount: monitor.monitoredStationIDs.count,
-                lastAlert: monitor.lastAlert
+                lastAlert: monitor.lastAlert,
+                onShowOnboarding: { showOnboarding = true }
             )
             .tabItem { Label("Alerts", systemImage: "bell.fill") }
         }
+        .fullScreenCover(isPresented: $showOnboarding) {
+            OnboardingView {
+                showOnboarding = false
+            }
+        }
         .onAppear {
-            locationManager.startForegroundTracking()
+            // Onboarding runs first on a fresh install — it owns the initial
+            // permission prompts. Location tracking starts once it's done.
+            if FuelStore.loadHasCompletedOnboarding() {
+                locationManager.startForegroundTracking()
+            } else {
+                showOnboarding = true
+            }
             monitor.update(stations: stations, favourites: refreshedFavourites,
                            fuel: selectedFuel, radiusKM: alertsRadius)
             monitor.setEnabled(alertsEnabled)
             // Refresh only when the cache is stale (twice-a-day policy).
             Task { await refresh() }
         }
+        .onChange(of: showOnboarding) { _, showing in
+            // After onboarding finishes (or the test re-run is dismissed),
+            // begin foreground location tracking if permission allows.
+            if !showing, FuelStore.loadHasCompletedOnboarding() {
+                locationManager.startForegroundTracking()
+            }
+        }
         .onChange(of: scenePhase) { _, newPhase in
             if newPhase == .active {
                 locationManager.startForegroundTracking()