fix duplicate 'let on' declaration in update_pump()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pool.js

@@ -183,7 +183,7 @@ function update_pump(temp, max, time) {
   }
 
   // compute the current switch state according to the schedule
-  let on = false;
+  on = false;
   if (schedule.length === 1) {
     on = true;
   }