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/AlertsView.swift

@@ -7,6 +7,7 @@ struct AlertsView: View {
     @Binding var radius: Double
     let monitoredCount: Int
     let lastAlert: String?
+    var onShowOnboarding: () -> Void = {}
 
     var body: some View {
         NavigationStack {
@@ -54,6 +55,19 @@ struct AlertsView: View {
                         }
                     }
                 }
+
+                // Testing hook — in production onboarding shows once at first
+                // launch; this button re-opens it to verify the flow.
+                Section {
+                    Button {
+                        onShowOnboarding()
+                    } label: {
+                        Label("Show onboarding (testing)", systemImage: "flag.fill")
+                            .font(.footnote)
+                    }
+                } footer: {
+                    Text("Testing only — onboarding normally appears once on first launch.")
+                }
             }
             .navigationTitle("Alerts")
         }