fix: stop broken background images from displaying (@mattlau1) (monkeytypegame#6526)

### Description

If the specified background image is invalid, then we can hide it using
`this.style.display='none'`, which will also hide the default white
outline and broken image icon

Also fixed a typo in the notification for it (temporairly ->
temporarily)

Tested fix by entering invalid custom background URL in settings (i.e.
`https://tehtuhgsfdsfjsdg.com/test.jpg`)

Closes monkeytypegame#6524 

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Thanks!! :)

Author: mattlau1

frontend/src/ts/controllers/theme-controller.ts

@@ -357,7 +357,7 @@ function applyCustomBackground(): void {
     img.setAttribute("src", Config.customBackground);
     img.setAttribute(
       "onError",
-      "javascript:window.dispatchEvent(new Event('customBackgroundFailed'))"
+      "javascript:this.style.display='none'; window.dispatchEvent(new Event('customBackgroundFailed'))"
     );
     container?.replaceChildren(img);
 
@@ -444,7 +444,7 @@ ConfigEvent.subscribe(async (eventKey, eventValue, nosave) => {
 
 window.addEventListener("customBackgroundFailed", () => {
   Notifications.add(
-    "Custom background link is either temporairly unavailable or expired. Please make sure the URL is correct or change it",
+    "Custom background link is either temporarily unavailable or expired. Please make sure the URL is correct or change it",
     0,
     { duration: 5 }
   );