fix(nav): Back onto a removed page also rewrites the entry to the feed

The fallback rendered the feed but left the history entry and the address bar on the dead page, so
a Feed click was a no-op (the same-page guard), filter mirroring kept `?page=messages` and the nav
diagnostics logged a page that does not exist. The entry is now replaced with the feed's, keeping
every other URL key (test red without it).

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

@@ -183,9 +183,13 @@ describe("NavigationProvider · Back", () => {
     const { result } = render();
     act(() => result.current.setPage("plex"));
     act(() => {
+      window.history.replaceState({ sfPage: "messages" }, "", "/?page=messages&keep=1");
       window.dispatchEvent(new PopStateEvent("popstate", { state: { sfPage: "messages" } }));
     });
     expect(result.current.page).toBe("feed");
+    // …and the entry says so too, so the address bar and a later Feed click agree with the screen.
+    expect(window.history.state).toMatchObject({ sfPage: "feed" });
+    expect(window.location.search).toBe("?keep=1");
   });
 });