feat: section 3 — km milestone notifications

- Foreground notification handler in App.tsx (iOS shows banners while active)
- requestNotificationPermissions() called on app mount
- GPS task tracks running distance per recording session (module-level state)
- Fires immediate notification at each km crossed, gated on kmNotifications setting

App.tsx

@@ -1,8 +1,25 @@
+import { useEffect } from 'react';
 import { StatusBar } from 'expo-status-bar';
+import * as Notifications from 'expo-notifications';
 import { GestureHandlerRootView } from 'react-native-gesture-handler';
 import { AppNavigator } from './src/navigation/AppNavigator';
+import { requestNotificationPermissions } from './src/services/gps';
+
+// Show notifications even when the app is in the foreground (iOS suppresses by default)
+Notifications.setNotificationHandler({
+  handleNotification: async () => ({
+    shouldShowBanner: true,
+    shouldShowList: true,
+    shouldPlaySound: true,
+    shouldSetBadge: false,
+  }),
+});
 
 export default function App() {
+  useEffect(() => {
+    requestNotificationPermissions();
+  }, []);
+
   return (
     <GestureHandlerRootView style={{ flex: 1 }}>
       <StatusBar style="light" />