fix: use relative API URL in CycleActions (nginx proxy compat)

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

frontend/src/pages/CycleActions.tsx

@@ -1,7 +1,7 @@
 import { useState, useEffect } from 'react'
 import { Play, CheckSquare, Clock, AlertCircle, ChevronDown, ChevronUp, Loader2 } from 'lucide-react'
 
-const API = 'http://localhost:8000'
+const API = ''
 
 interface ActionDef {
   id: string