fix: sportsplusrower use Concept2 watt formula instead of device byte

CARE Jet 600 rower (CARE10692135) reports only 8-17W via BLE (byte[10] BCD),
which is unrealistically low. Replace with the Concept2 rowing power formula
used in the rower base class: Watts = 2.8 * (500 / paceSeconds)^3.
Speed is already derived from cadence SPM, pace is computed as 1800/speed_km_h.

Also move cadence/speed update before watt calculation so watts use the
fresh speed value from the current 0x10 subtype packet.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: cagnulein

src/devices/sportsplusrower/sportsplusrower.cpp

@@ -136,15 +136,20 @@ void sportsplusrower::characteristicChanged(const QLowEnergyCharacteristic &char
     double kcal = 0;
     bool cadence_eval = false;
 
-    m_watt = ((newValue.at(10) >> 4) * 10) + (newValue.at(10) & 0x0F);
-    emit debug(QStringLiteral("Current watt: ") + QString::number(m_watt.value()));
-
     if(newValue.at(1) == 0x10) {
         Cadence = ((newValue.at(3) >> 4) * 10) + (newValue.at(3) & 0x0F);
         Speed = 0.37497622 * ((double)Cadence.value());
         emit debug(QStringLiteral("Current speed: ") + QString::number(Speed.value()));
     }
 
+    if (Speed.value() > 0) {
+        double paceSeconds = 1800.0 / Speed.value();
+        m_watt = rower::calculateWattsFromPace(paceSeconds);
+    } else {
+        m_watt = 0;
+    }
+    emit debug(QStringLiteral("Current watt: ") + QString::number(m_watt.value()));
+
     if (!firstCharChanged) {
         double elapsedMs = lastTimeCharChanged.msecsTo(now);
         Distance += ((Speed.value() / 3600.0) / (1000.0 / elapsedMs));