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.

include/config.h

@@ -40,6 +40,12 @@
 // Current logging interval for data collection
 #define CURRENT_LOG_INTERVAL_MS 100  // Log current readings every 100ms
 
+// Simple Stall Detection
+// Based on clean data: Running ~2A, Stall ~17A (8.5x difference)
+#define STALL_THRESHOLD 8.0f          // Amps - midpoint between 2A run and 17A stall
+#define STALL_CONFIRM_SAMPLES 3       // Debounce: 3 samples = 300ms at 100ms interval
+#define STALL_STABILIZE_MS 500        // Ignore current spikes for 500ms after direction change
+
 // Web Server
 #define HTTP_PORT 80