fix(nav): Back onto a page this build no longer has lands on the feed

A history entry pushed while the Messages page existed still names it after a reload onto the new
build; popstate took sfPage unchecked and the shell's module lookup threw. It is now validated with
isPage, like every initial-load path already was.

frontend/src/components/NavigationProvider.dom.test.tsx

@@ -176,6 +176,19 @@ describe("NavigationProvider · setPage's same-page guard", () => {
   });
 });
 
+describe("NavigationProvider · Back", () => {
+  // A tab that had a since-removed module open, reloaded onto the new build: its older history
+  // entries still name that page, and Back lands on one of them.
+  it("lands on the feed when the entry names a page this build does not have", () => {
+    const { result } = render();
+    act(() => result.current.setPage("plex"));
+    act(() => {
+      window.dispatchEvent(new PopStateEvent("popstate", { state: { sfPage: "messages" } }));
+    });
+    expect(result.current.page).toBe("feed");
+  });
+});
+
 describe("NavigationProvider · never under an open overlay", () => {
   it("setPage closes the overlay first and pushes only once its entry is reclaimed", async () => {
     const { result } = render();

frontend/src/components/NavigationProvider.tsx

@@ -2,7 +2,7 @@ import { useCallback, useEffect, useMemo, useState, type ReactNode } from "react
 import { useLatest } from "../lib/useLatest";
 import { useTranslation } from "react-i18next";
 import { useMe } from "../lib/useMe";
-import { urlWithParams, type Page } from "../lib/urlState";
+import { isPage, urlWithParams, type Page } from "../lib/urlState";
 import { LOGS_KEYS } from "../lib/logsUrlState";
 import { pageTitleKey } from "../lib/pageMeta";
 import { useDocumentObjectTitle } from "../lib/documentTitle";
@@ -197,7 +197,10 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
       window.history.replaceState({ ...rest, sfPage: mountPage }, "", url);
     }
     function onPop(e: PopStateEvent) {
-      const p = (e.state?.sfPage as Page) ?? "feed";
+      // An entry can name a page this build no longer has (one pushed before a module was removed,
+      // reached by Back after a reload onto the new build); that lands on the feed, not on a crash.
+      const sf: unknown = e.state?.sfPage;
+      const p: Page = typeof sf === "string" && isPage(sf) ? sf : "feed";
       setPageState(p);
       setAccountRaw(PAGE_KEY, p);
       // The YouTube-search sub-view and the channel page ride in history.state, so Back/Forward