test(channels): cover the tab-history logic; memoize viewTabs (S2b review)

Addresses the 2 applied findings from /code-review high on 735d08c:
- Add ChannelsProvider.dom.test.tsx (4 tests): setView pushes a _chTab history entry +
  persists the tab, re-selecting the current tab pushes nothing, a genuine popstate carrying
  _chTab restores + re-persists it, and a popstate without _chTab is ignored. Real storage
  (jsdom) with the account pinned to 1; only NavigationProvider is mocked. vitest 120->124.
- Memoize viewTabs (keyed on the translation fn) so it isn't rebuilt on every Channels render.

The third finding (consolidate the hand-rolled single-select sync chips with <Tabs>) is
deferred by design into the S2c plan, where MatchToggle will be a shared segmented-control
primitive that can also absorb the sync chips.

gate-green: tsc + vitest 124 + eslint-0 + knip + prettier.

frontend/src/components/Channels.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useRef, useState } from "react";
+import { useEffect, useMemo, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
@@ -432,9 +432,14 @@ export default function Channels() {
   }, [filters, search, sort, resetPage]);
   const cardPager = <Pager {...pagerProps} />;
 
-  const viewTabs: TabDef[] = (
-    ["subscribed", "discovery", "hidden", "blocked"] as ChannelsView[]
-  ).map((v) => ({ id: v, label: t(`channels.tabs.${v}`) }));
+  const viewTabs: TabDef[] = useMemo(
+    () =>
+      (["subscribed", "discovery", "hidden", "blocked"] as ChannelsView[]).map((v) => ({
+        id: v,
+        label: t(`channels.tabs.${v}`),
+      })),
+    [t],
+  );
   const tabs = (
     <div className="px-4 pt-4 max-w-384 mx-auto flex items-center justify-between gap-4 flex-wrap">
       <Tabs

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

@@ -0,0 +1,73 @@
+// @vitest-environment jsdom
+//
+// ChannelsProvider's tab-history behaviour: setView pushes a `_chTab` history entry + persists the tab,
+// and a genuine browser Back (popstate carrying `_chTab`) restores + re-persists it. This wiring was
+// re-routed through usePersistedTab in R13 S2b, so it's exercised here rather than only by hand E2E.
+//
+// Uses REAL storage (jsdom localStorage/sessionStorage) with the tab's account pinned to 1, so
+// usePersistedTab / useSortState / useFilterState all resolve their account-scoped keys; only
+// NavigationProvider (the sole runtime dependency) is mocked, pinned to the channels page.
+import { describe, it, expect, beforeEach, vi } from "vitest";
+import { renderHook, act } from "@testing-library/react";
+import type { ReactNode } from "react";
+
+vi.mock("./NavigationProvider", () => ({
+  useNavigation: () => ({ page: "channels" }),
+  useNavigationActions: () => ({ setPage: vi.fn() }),
+}));
+
+import { ChannelsProvider, useChannels, useChannelsActions } from "./ChannelsProvider";
+
+const wrapper = ({ children }: { children: ReactNode }) => (
+  <ChannelsProvider>{children}</ChannelsProvider>
+);
+const render = () => renderHook(() => ({ ...useChannels(), ...useChannelsActions() }), { wrapper });
+const VIEW_KEY = "siftlode.channelsView.1"; // account-scoped (accountKey suffixes the pinned id)
+
+beforeEach(() => {
+  localStorage.clear();
+  sessionStorage.setItem("siftlode.activeAccount", "1"); // pin the tab's account
+  window.history.replaceState({ sfPage: "channels" }, ""); // setView only owns an entry on this page
+});
+
+describe("ChannelsProvider tab history", () => {
+  it("setView pushes a _chTab history entry and persists the tab", () => {
+    const { result } = render();
+    expect(result.current.view).toBe("subscribed"); // default
+
+    act(() => result.current.setView("hidden"));
+
+    expect(result.current.view).toBe("hidden");
+    expect(window.history.state).toMatchObject({ sfPage: "channels", _chTab: "hidden" });
+    expect(localStorage.getItem(VIEW_KEY)).toBe("hidden");
+  });
+
+  it("does NOT push a new entry when re-selecting the current tab", () => {
+    const { result } = render();
+    const spy = vi.spyOn(window.history, "pushState");
+    act(() => result.current.setView("subscribed")); // already the current tab
+    expect(spy).not.toHaveBeenCalled();
+    spy.mockRestore();
+  });
+
+  it("a genuine Back (popstate carrying _chTab) restores and re-persists that tab", () => {
+    const { result } = render();
+    act(() => result.current.setView("blocked"));
+    expect(result.current.view).toBe("blocked");
+
+    // Browser Back to an entry stamped with a different tab.
+    act(() =>
+      window.dispatchEvent(new PopStateEvent("popstate", { state: { _chTab: "discovery" } })),
+    );
+
+    expect(result.current.view).toBe("discovery");
+    expect(localStorage.getItem(VIEW_KEY)).toBe("discovery");
+  });
+
+  it("ignores a popstate for an entry outside the manager (no _chTab)", () => {
+    const { result } = render();
+    act(() => result.current.setView("hidden"));
+    act(() => window.dispatchEvent(new PopStateEvent("popstate", { state: { sfPage: "feed" } })));
+    expect(result.current.view).toBe("hidden"); // unchanged
+  });
+});