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