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

Shared/FuelStore.swift

@@ -182,6 +182,7 @@ struct FuelStore {
     static let favouritesKey = "fuelboard.favourites"      // [FuelStation] JSON
     static let alertsEnabledKey = "fuelboard.alertsEnabled" // Bool
     static let alertsRadiusKey = "fuelboard.alertsRadius"   // Double km
+    static let onboardingCompletedKey = "fuelboard.onboardingCompleted" // Bool
     static let lastRefreshKey = "fuelboard.lastRefresh"      // TimeInterval (seconds since 1970)
 
     // MARK: Stations
@@ -358,6 +359,18 @@ struct FuelStore {
         return Date().timeIntervalSince(last) < refreshInterval
     }
 
+    // MARK: Onboarding — the app shows the intro screen on first launch only
+    // (a test button in the Alerts tab re-opens it). Stored in the app group
+    // so the widget can see it too if ever needed.
+
+    static func loadHasCompletedOnboarding() -> Bool {
+        UserDefaults(suiteName: appGroupSuite)?.bool(forKey: onboardingCompletedKey) ?? false
+    }
+
+    static func saveHasCompletedOnboarding(_ completed: Bool) {
+        UserDefaults(suiteName: appGroupSuite)?.set(completed, forKey: onboardingCompletedKey)
+    }
+
     // MARK: Low-level keychain helpers
 
     private static func keychainData(service: String) -> Data? {