add is_system to analog sensor so internal sensors cannot be removed

interface/src/app/main/SensorsAnalogDialog.tsx

@@ -447,6 +447,7 @@ const SensorsAnalogDialog = ({
           <Box flexGrow={1} sx={{ '& button': { mt: 0 } }}>
             <Button
               startIcon={<RemoveIcon />}
+              disabled={editItem.s}
               variant="outlined"
               color="warning"
               onClick={remove}

interface/src/app/main/types.ts

@@ -95,6 +95,7 @@ export interface AnalogSensor {
   f: number;
   t: number;
   d: boolean; // deleted flag
+  s: boolean; // system customization flag
   o_n?: string;
 }