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.

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "productName": "Bocken",
   "identifier": "org.bocken.app",
-  "version": "0.3.0",
+  "version": "0.4.0",
   "build": {
     "devUrl": "http://192.168.1.4:5173",
     "frontendDist": "https://bocken.org"