fix: go back a page when deleting the last page

Fixes #98

Author: chrrs

src/main/java/me/chrr/scribble/screen/ScribbleBookEditScreen.java

@@ -505,6 +505,9 @@ public void insertPageAt(int page, @Nullable RichText content) {
     public void deletePage(int page) {
         this.pages.remove(page);
         this.dirty = true;
+
+        if (page >= this.pages.size() - 1)
+            this.showPage(page - 1, false);
         this.updateCurrentPages();
     }
     //endregion