gps related minor fixes

pi/backend/main.py

@@ -139,6 +139,10 @@ def on_arduino_data(data):
     # Always include current GPIO state (UI dedupes)
     data = dict(data)  # Don't mutate original
     data["theme_switch"] = gpio.theme_switch
+    
+    # backend voltage offset correction
+    if "voltage" in data:
+        data["voltage"] += 0.2  # Calibration offset
 
     def emit_fn(d):
         socketio.emit("arduino", d)