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" />

CLAUDE.md

@@ -112,10 +112,10 @@ Items below are what remains before v1 is shippable.
 - [x] **Cadence CSC parsing** — stateful parsing in `parseCscMeasurement()`: tracks previous crank revolution count + event time (uint16, 1/1024 s), computes RPM from delta with uint16 rollover handling
 - [x] **BLE persistence** — `savePairedDevice` / `loadPairedDevices` / `removePairedDevice` in `ble.ts` via AsyncStorage; SensorPairingScreen shows saved sensors at top, auto-attempts reconnect on mount, Forget button removes a device
 
-### 3 — Km notifications (half day)
+### 3 — Km notifications ✅
 
-- [ ] **Permission request** — call `Notifications.requestPermissionsAsync()` on first launch
-- [ ] **Milestone tracker** — track last notified km in the GPS background task (or store); fire a notification each time `distanceMeters` crosses a new km boundary, gated on the `kmNotifications` setting from AsyncStorage
+- [x] **Permission request** — `requestNotificationPermissions()` called in `App.tsx` on mount; foreground notification handler set at module level so iOS shows banners while app is active
+- [x] **Milestone tracker** — module-level `runningDistanceMeters` / `lastNotifiedKm` / `prevPoint` in `gps.ts`, reset on each `startGpsRecording()`; each incoming location adds haversine distance and fires a notification when a new km is crossed, gated on the `kmNotifications` AsyncStorage setting
 
 ### 4 — Android battery optimization prompt (1–2 hours)
 

src/services/gps.ts

@@ -1,9 +1,16 @@
 import * as Location from 'expo-location';
 import * as TaskManager from 'expo-task-manager';
+import * as Notifications from 'expo-notifications';
+import AsyncStorage from '@react-native-async-storage/async-storage';
 import { useRecordingStore } from '../store/recording';
 
 const BACKGROUND_LOCATION_TASK = 'background-location-task';
 
+// Module-level km milestone state — reset each recording
+let runningDistanceMeters = 0;
+let lastNotifiedKm = 0;
+let prevPoint: { lat: number; lon: number } | null = null;
+
 TaskManager.defineTask(BACKGROUND_LOCATION_TASK, async ({ data, error }: any) => {
   if (error) return;
   const locations: Location.LocationObject[] = data?.locations ?? [];
@@ -11,7 +18,7 @@ TaskManager.defineTask(BACKGROUND_LOCATION_TASK, async ({ data, error }: any) =>
   if (status !== 'recording') return;
 
   for (const loc of locations) {
-    addTrackPoint({
+    const point = {
       lat: loc.coords.latitude,
       lon: loc.coords.longitude,
       ele: loc.coords.altitude ?? 0,
@@ -19,10 +26,35 @@ TaskManager.defineTask(BACKGROUND_LOCATION_TASK, async ({ data, error }: any) =>
       hr: ble.hr,
       power: ble.power,
       cad: ble.cadence,
-    });
+    };
+    addTrackPoint(point);
+
+    if (prevPoint) {
+      runningDistanceMeters += haversineMeters(prevPoint, point);
+      await maybeNotifyKm(runningDistanceMeters);
+    }
+    prevPoint = point;
   }
 });
 
+async function maybeNotifyKm(distanceMeters: number): Promise<void> {
+  const kmSetting = await AsyncStorage.getItem('kmNotifications');
+  if (kmSetting === 'false') return;
+
+  const crossedKm = Math.floor(distanceMeters / 1000);
+  if (crossedKm <= lastNotifiedKm) return;
+  lastNotifiedKm = crossedKm;
+
+  await Notifications.scheduleNotificationAsync({
+    content: {
+      title: `${crossedKm} km`,
+      body: `You've covered ${crossedKm} km!`,
+      sound: true,
+    },
+    trigger: null,
+  });
+}
+
 export async function requestLocationPermissions(): Promise<boolean> {
   const { status: fg } = await Location.requestForegroundPermissionsAsync();
   if (fg !== 'granted') return false;
@@ -31,6 +63,11 @@ export async function requestLocationPermissions(): Promise<boolean> {
 }
 
 export async function startGpsRecording(): Promise<void> {
+  // Reset km tracking for this recording session
+  runningDistanceMeters = 0;
+  lastNotifiedKm = 0;
+  prevPoint = null;
+
   await Location.startLocationUpdatesAsync(BACKGROUND_LOCATION_TASK, {
     accuracy: Location.Accuracy.BestForNavigation,
     timeInterval: 1000,
@@ -47,3 +84,19 @@ export async function stopGpsRecording(): Promise<void> {
   const isRunning = await Location.hasStartedLocationUpdatesAsync(BACKGROUND_LOCATION_TASK);
   if (isRunning) await Location.stopLocationUpdatesAsync(BACKGROUND_LOCATION_TASK);
 }
+
+export async function requestNotificationPermissions(): Promise<void> {
+  await Notifications.requestPermissionsAsync();
+}
+
+function haversineMeters(a: { lat: number; lon: number }, b: { lat: number; lon: number }): number {
+  const R = 6371000;
+  const dLat = toRad(b.lat - a.lat);
+  const dLon = toRad(b.lon - a.lon);
+  const sinLat = Math.sin(dLat / 2);
+  const sinLon = Math.sin(dLon / 2);
+  const c = sinLat * sinLat + Math.cos(toRad(a.lat)) * Math.cos(toRad(b.lat)) * sinLon * sinLon;
+  return R * 2 * Math.atan2(Math.sqrt(c), Math.sqrt(1 - c));
+}
+
+const toRad = (deg: number) => (deg * Math.PI) / 180;