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/webserver.cpp

@@ -109,6 +109,19 @@ const char index_html[] PROGMEM = R"rawliteral(
             border-top: 2px solid #0f3460;
             margin: 30px 0;
         }
+        .stall-warning {
+            background: #ff5252;
+            color: white;
+            padding: 15px;
+            border-radius: 10px;
+            margin-bottom: 20px;
+            font-weight: bold;
+            font-size: 18px;
+            display: none;
+        }
+        .stall-warning.active {
+            display: block;
+        }
         .pingpong-section {
             background: #16213e;
             padding: 20px;
@@ -169,6 +182,8 @@ const char index_html[] PROGMEM = R"rawliteral(
     <div class="container">
         <h1>Motor Control</h1>
         
+        <div class="stall-warning" id="stallWarning">⚠️ STALL DETECTED!</div>
+        
         <div class="current-display">
             <div>
                 <div class="current-label">CURRENT R</div>
@@ -323,6 +338,14 @@ const char index_html[] PROGMEM = R"rawliteral(
                     document.getElementById('currentR').textContent = data.currentR.toFixed(2);
                     document.getElementById('currentL').textContent = data.currentL.toFixed(2);
                     
+                    // Update stall warning
+                    const stallWarning = document.getElementById('stallWarning');
+                    if (data.stalled) {
+                        stallWarning.classList.add('active');
+                    } else {
+                        stallWarning.classList.remove('active');
+                    }
+                    
                     // Update pingpong status
                     if (data.pingpong) {
                         ppStatus.textContent = 'ACTIVE';
@@ -387,6 +410,7 @@ void handleStatus() {
                   ",\"direction\":" + String(motor.getDirection()) +
                   ",\"currentR\":" + String(motor.getCurrentRight(), 2) +
                   ",\"currentL\":" + String(motor.getCurrentLeft(), 2) +
+                  ",\"stalled\":" + (motor.isStalled() ? "true" : "false") +
                   ",\"pingpong\":" + (motor.isPingpongActive() ? "true" : "false") +
                   ",\"ppSpeed\":" + String(motor.getPingpongSpeed()) +
                   ",\"ppTime\":" + String(motor.getPingpongTime()) +