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();