feat: record cadence from step detector during GPS workouts

Use Android TYPE_STEP_DETECTOR sensor in LocationForegroundService to
count steps in a 15s rolling window. Cadence (spm) is computed at each
GPS point and stored alongside lat/lng/altitude/speed. Session detail
page shows cadence chart when data is available.

No additional permissions required — step detector is not a restricted
sensor. Gracefully skipped on devices without the sensor.

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "homepage",
-	"version": "1.25.1",
+	"version": "1.25.2",
 	"private": true,
 	"type": "module",
 	"scripts": {