onboarding: persist completed flag keychain-first so it doesn't re-show

The completed flag lived only in app-group UserDefaults
(UserDefaults(suiteName: appGroupSuite)). On free SideStore accounts the
app-group container isn't provisioned, so the suite read as nil: save was a
silent no-op and load always returned false -> onboarding re-appeared on
every launch even after finishing and granting all permissions.

Switch to saveString/loadString (keychain-first, app-group mirror) under
onboardingCompletedKey, matching favourites/distance-unit persistence which
survives reinstall and works without a provisioning group. loadString falls
back to the app-group mirror, so anyone who previously completed on a
provisioned group keeps their flag; users on unprovisioned groups complete
once more and then it sticks.

Shared/FuelStore.swift

@@ -934,15 +934,19 @@ struct FuelStore {
     }
 
     // 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.
+    // (a test button in the Alerts tab re-opens it). Stored KEYCHAIN-FIRST
+    // (with an app-group mirror) for the same reason as favourites/distance
+    // unit: free SideStore accounts don't provision the app-group container,
+    // so an app-group-only flag silently fails to save AND reloads as false,
+    // making onboarding re-appear on every launch. Keychain survives reinstall
+    // and is shared with the extension.
 
     static func loadHasCompletedOnboarding() -> Bool {
-        UserDefaults(suiteName: appGroupSuite)?.bool(forKey: onboardingCompletedKey) ?? false
+        (loadString(service: onboardingCompletedKey) ?? "0") == "1"
     }
 
     static func saveHasCompletedOnboarding(_ completed: Bool) {
-        UserDefaults(suiteName: appGroupSuite)?.set(completed, forKey: onboardingCompletedKey)
+        saveString(completed ? "1" : "0", service: onboardingCompletedKey)
     }
 
     // MARK: Low-level keychain helpers