add image cache function

assets/ts/main.ts

@@ -3,8 +3,10 @@ import { trackMouseInit } from './trackMouse'
 import { thresholdCtlInit } from './thresholdCtl'
 import { imagesArrayLen } from './dataFetch'
 import { vwRefreshInit } from './overlay'
+import { preloader } from './imageCache'
 
 function init(): void {
+  preloader(0)
   vwRefreshInit()
   imgIndexSpanUpdate(0, imagesArrayLen)
   thresholdCtlInit()