Add PWM motor control and refactor engine update

Introduce ENA pins for both motors and implement rpmToPwm and setMotor.
Replace extensive Serial debug prints with concise motor‑drive logic in
Engine::update. Consolidate sonar pins into a single TRIG_AND_ECHO_PIN.
Update pinMode calls in the sketch to configure the new ENA pins.

Engine.h

@@ -36,6 +36,9 @@ public:
   int getRPM() const;
   int getSpeedKmh() const;
 
+  uint8_t rpmToPwm(float rpm);
+  void setMotor(uint8_t inA, uint8_t inB, uint8_t ena, float rpm, bool reverse = false);
+
 private:
   const uint8_t _throttlePin;
   const uint8_t _clutchPin;