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

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)