Add pingpong mode with speed, time, and randomness settings

- Add pingpong oscillation mode that alternates motor direction
- Settings: base speed (10-100%), time before return (0.5-10s)
- Randomness controls for both speed (0-50%) and time (0-50%)
- Web UI with sliders and start/stop buttons
- API endpoints: /pingpong/start and /pingpong/stop
- Real-time status polling shows pingpong active state

include/motor.h

@@ -21,6 +21,15 @@ public:
     
     // Callback for stall events
     void setStallCallback(void (*callback)(float current));
+    
+    // Pingpong mode
+    void startPingpong(int speed, int timeMs, int speedRandomPercent, int timeRandomPercent);
+    void stopPingpong();
+    bool isPingpongActive();
+    int getPingpongSpeed();
+    int getPingpongTime();
+    int getPingpongSpeedRandom();
+    int getPingpongTimeRandom();
 
 private:
     int _speed = 0;
@@ -31,9 +40,22 @@ private:
     unsigned long _stallStartTime = 0;
     void (*_stallCallback)(float current) = nullptr;
     
+    // Pingpong state
+    bool _pingpongActive = false;
+    int _pingpongBaseSpeed = 50;
+    int _pingpongBaseTime = 2000;
+    int _pingpongSpeedRandomPercent = 0;
+    int _pingpongTimeRandomPercent = 0;
+    int _pingpongCurrentSpeed = 50;
+    int _pingpongCurrentTime = 2000;
+    unsigned long _pingpongLastSwitch = 0;
+    int _pingpongDirection = 1;
+    
     void applyMotorState();
     float readCurrentSense(int pin);
     void checkStall();
+    void updatePingpong();
+    int applyRandomness(int baseValue, int randomPercent);
 };
 
 extern MotorController motor;