Make car mass configurable at runtime

Replace the compile‑time CAR_MASS constant with a runtime‑settable
_member _carMass. Add Engine::setCarMass, default it to 1200 kg, and
use it in the acceleration calculation. Prompt the user for the
car mass on startup via Serial and update the enable/disable messages.

Engine.cpp

@@ -33,7 +33,7 @@ void Engine::update(float dt, uint8_t gearIdx) {
   const float dragForce = 0.5f * DRAG_COEFF * FRONTAL_AREA * (_speedMs * _speedMs);
   const int direction = (gearIdx == -1) ? -1 : 1;
   const float netForce = direction * engineForce - brakeForce - dragForce;
-  const float acceleration = netForce / CAR_MASS;
+  const float acceleration = netForce / _carMass;
 
   _speedMs += acceleration * dt;
   if (_speedMs < 0.0f) _speedMs = 0.0f;