fix: custom scheduling

`postBaseUrl` will be necessary until the backend isn't updated.

Reproduction steps:
1. Add some JS to the Deck's custom scheduling. e.g. `console.log("foo")`
2. Open a deck
3. See `ReferenceError` in the console or logcat

AnkiDroid/src/main/java/com/ichi2/anki/AbstractFlashcardViewer.kt

@@ -2263,6 +2263,8 @@ abstract class AbstractFlashcardViewer :
         override fun onPageStarted(view: WebView?, url: String?, favicon: Bitmap?) {
             pageRenderStopwatch.reset()
             pageFinishedFired = false
+            val script = "globalThis.ankidroid = globalThis.ankidroid || {}; ankidroid.postBaseUrl = ``"
+            view?.evaluateJavascript(script, null)
         }
 
         override fun shouldInterceptRequest(