fix: Correct Modbus client class name in signals endpoint

Use correct class name 'ModbusClient' instead of 'ModbusTCPClient'

src/dashboard/api.py

@@ -718,10 +718,10 @@ async def get_signals():
     
     # Try to use real protocol data for Modbus, fallback for OPC UA
     try:
-        from .protocol_clients import ModbusTCPClient
+        from .protocol_clients import ModbusClient
         
         # Create Modbus client
-        modbus_client = ModbusTCPClient(
+        modbus_client = ModbusClient(
             host="localhost",
             port=502,
             unit_id=1