Remove pingpong mode completely

- Remove pingpong method declarations and member variables from motor.h
- Remove pingpong implementation (startPingpong, stopPingpong, updatePingpong, etc.) from motor.cpp
- Remove pingpong UI section, handlers, and routes from webserver.cpp
- Remove stopPingpong call from stall callback in main.cpp

src/main.cpp

@@ -8,7 +8,6 @@
 void onMotorStall() {
     Serial.println("Stall callback triggered - stopping motor!");
     motor.stop();
-    motor.stopPingpong();
 }
 
 void setupWiFi() {