android: native GPS tracking with foreground service for screen-off support

Move GPS collection from WebView JS (watchPosition) to native Android
LocationForegroundService, which survives screen-off. JS polls native
side for accumulated points. Also: auto-enable GPS for cardio exercises,
filter saved track to workout duration only, fix live map batch updates,
notification tap opens active workout, and fix build script for pnpm.

src/routes/fitness/[workout=fitnessWorkout]/[active=fitnessActive]/+page.svelte

@@ -119,6 +119,7 @@
 	$effect(() => {
 		const len = gps.track.length;
 		if (len > prevTrackLen && liveMap && gps.latestPoint) {
+			// Add all new points since last update (native polling delivers batches)
 			for (let i = prevTrackLen; i < len; i++) {
 				const p = gps.track[i];
 				livePolyline.addLatLng([p.lat, p.lng]);