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.h

@@ -13,7 +13,6 @@ constexpr float GEAR_RATIO[6] = {
 
 constexpr float FINAL_DRIVE = 3.74f;   // przekładnia różnicowa
 constexpr float WHEEL_RADIUS = 0.30f;  // m
-constexpr float CAR_MASS = 1200.0f;    // kg
 constexpr float RPM_IDLE = 800.0f;     // obroty jałowe
 constexpr float RPM_MAX = 7000.0f;     // maksymalne obroty
 
@@ -31,6 +30,8 @@ public:
 
   void begin();
 
+  void setCarMass(float mass) { _carMass = mass; }
+
   void update(float dt, uint8_t gearIdx);
 
   int getRPM() const;
@@ -46,6 +47,7 @@ private:
 
   float _engineRPM = EngineConsts::RPM_IDLE;
   float _speedMs = 0.0f;
+  float _carMass = 1200.0f;
 
   float readNormalized(uint8_t pin) const;
 };