refactor(channels): adopt shared Tabs + usePersistedTab for the manager tab row (R13 S2b)

Retire the hand-rolled channel-manager view pills for the shared <Tabs> component (given an
optional className so it embeds in the justify-between header without the default mb-4), and
back view + layout on usePersistedTab instead of manual getAccountRaw/setAccountRaw useState.
That subsumes the account-pin re-hydrate (usePersistedTab now re-reads on the pin), so
ChannelsProvider's bespoke markManagerDirty/useAccountRehydrate is gone; setView keeps the
history-tab logic (_chTab pushState + popstate) layered on the stable raw setter, and
useAccountPersistedState's setter is now memoized so that stable action holds.

Per the user's call, unify every channel-manager pill on the rounded-lg idiom: the view tabs
via <Tabs>, the sync chips and tag chips + the "manage tags" affordance from rounded-full to
rounded-lg — no more odd-one-out rounded-full.

gate-green: tsc + vitest 120 + eslint-0 + knip + prettier. Self-E2E'd: tab switch, browser
Back across tabs (Blocked->Hidden->Subscriptions with localStorage in sync), and a fresh tab
restoring view=hidden on the FIRST load; zero console errors.

frontend/src/lib/storage.ts

@@ -159,11 +159,16 @@ export function useAccountPersistedState(
 ): [string, (value: string) => void] {
   const [value, setValue] = useState<string>(() => getAccountRaw(base) ?? fallback);
   const markDirty = useAccountRehydrate(() => setValue(getAccountRaw(base) ?? fallback));
-  const set = (next: string) => {
-    markDirty();
-    setValue(next);
-    setAccountRaw(base, next);
-  };
+  // Stable setter (base + markDirty are both stable) so a provider can build a stable action on top
+  // of it — e.g. ChannelsProvider's setView, which must not change identity on every tab switch.
+  const set = useCallback(
+    (next: string) => {
+      markDirty();
+      setValue(next);
+      setAccountRaw(base, next);
+    },
+    [base, markDirty],
+  );
   return [value, set];
 }