refactor: fallback missing pages to home page (#4600)

See
https://discord.com/channels/955905230107738152/1318274135679438959/1318274135679438959

With undo current selected pages can be deleted
though we do not have a logic reverting selected page or instance.

So here is quick fix to always fallback to home page if pageId in url
references missing page.

apps/builder/app/shared/pages/use-switch-page.ts

@@ -12,14 +12,14 @@ import {
 } from "~/shared/nano-states";
 import { builderPath } from "~/shared/router-utils";
 import { $selectedPage, selectPage } from "../awareness";
+import { findPageByIdOrPath } from "@webstudio-is/sdk";
 
 const setPageStateFromUrl = () => {
   const searchParams = new URLSearchParams(window.location.search);
   const pages = $pages.get();
   if (pages === undefined) {
     return;
   }
-  const pageId = searchParams.get("pageId") ?? pages.homePage.id;
 
   let mode = searchParams.get("mode");
 
@@ -30,6 +30,12 @@ const setPageStateFromUrl = () => {
 
   setBuilderMode(mode);
 
+  // check the page actually exists
+  // to avoid confusing the user with broken state
+  const pageId =
+    findPageByIdOrPath(searchParams.get("pageId") ?? "", pages)?.id ??
+    pages.homePage.id;
+
   $selectedPageHash.set(searchParams.get("pageHash") ?? "");
   selectPage(pageId);
 };