feat: add simple stall detection with threshold + debounce

- Add STALL_THRESHOLD (8A) and STALL_CONFIRM_SAMPLES (3) config
- Add 500ms stabilization delay after direction change to prevent false positives
- Add stall warning banner in web UI
- Stop motor and pingpong when stall detected

Algorithm: If active current > 8A for 3 consecutive samples (300ms),
and motor has been running for at least 500ms, trigger stall callback.

src/main.cpp

@@ -4,6 +4,13 @@
 #include "motor.h"
 #include "webserver.h"
 
+// Called when motor stall is detected
+void onMotorStall() {
+    Serial.println("Stall callback triggered - stopping motor!");
+    motor.stop();
+    motor.stopPingpong();
+}
+
 void setupWiFi() {
     Serial.println("Connecting to WiFi...");
     
@@ -39,11 +46,12 @@ void setup() {
     
     Serial.println("\n=============================");
     Serial.println("  BTS7960 Motor Controller");
-    Serial.println("  (Stall Detection Removed)");
+    Serial.println("  Simple Stall Detection");
     Serial.println("=============================\n");
     
     // Initialize motor controller
     motor.begin();
+    motor.setStallCallback(onMotorStall);
     
     // Connect to WiFi
     setupWiFi();