fix(fitness): request ACTIVITY_RECOGNITION for cadence

Android step detector silently returns no events on API 29+
when ACTIVITY_RECOGNITION is ungranted, so cadence was always
absent from recorded tracks. Declare the permission, request
it at GPS start, guard sensor registration and retry it from
MainActivity.onRequestPermissionsResult when the user grants
mid-session, and toast a hint if they deny.

src-tauri/gen/android/app/src/main/AndroidManifest.xml

@@ -9,6 +9,8 @@
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
     <uses-permission android:name="android.permission.CAMERA" />
     <uses-permission android:name="android.permission.VIBRATE" />
+    <!-- Step detector sensor (cadence during GPS workouts); runtime-requested on API 29+ -->
+    <uses-permission android:name="android.permission.ACTIVITY_RECOGNITION" />
 
     <!-- AndroidTV support -->
     <uses-feature android:name="android.software.leanback" android:required="false" />