feat(ios): apply cadence_gain and cadence_offset in Apple Watch cadence path

The cadence_gain and cadence_offset settings are already exposed in
the UI and applied by other device drivers (Proform, Domyos, etc.),
but were not applied when Cadence is sourced from Apple Watch step
cadence. This made the settings dead-weight for users on the Fake
Treadmill / Apple Watch path.

Wires up the existing settings so users can scale or offset the
broadcast Cadence (useful when receiving apps interpret BLE FTMS
cadence with different scaling than QZ broadcasts). Defaults are
no-ops (gain=1, offset=0). Speed calculation still uses raw
cadence so the Wheel Ratio behaves identically.

Author: miraicat

src/devices/treadmill.cpp

@@ -622,7 +622,13 @@ bool treadmill::cadenceFromAppleWatch() {
                            QZSettings::default_fakedevice_treadmill).toBool();
         if (appleWatchCadence > 0) {
             evaluateStepCount();
-            Cadence = appleWatchCadence;
+            const double cadenceGain =
+                settings.value(QZSettings::cadence_gain,
+                               QZSettings::default_cadence_gain).toDouble();
+            const double cadenceOffset =
+                settings.value(QZSettings::cadence_offset,
+                               QZSettings::default_cadence_offset).toDouble();
+            Cadence = (appleWatchCadence * cadenceGain) + cadenceOffset;
             if (appleWatchTreadmillSpeed) {
                 const double ratio =
                     settings.value(QZSettings::cadence_sensor_speed_ratio,