refactor: component files export only components; lint fails on a warning

The 43 react-refresh/only-export-components warnings the gate let through meant an edit to a
provider (or AnchoredPopover, Toaster, plexDetailUi, …) could not be hot-swapped: Fast Refresh
reloads a module that also exports hooks or helpers.

- Each provider's contexts, the types its hooks return and the hooks themselves live in a sibling
  context module (navigationContext.ts, channelsContext.ts, …); the provider file keeps the
  component. loadAccountFilters moved next to the feed filters' defaults (feedFiltersInit.ts),
  following navigationInit/playlistsInit.
- Helpers that sat in component files have their own modules: welcomeMode, downloadTitle,
  plexPlayable, channelSync, ui/anchoredPopoverHooks, ui/pageToolbarSlot, ui/pagerContent,
  ui/toastLevels, ui/overlayOrder, ui/tableAlign, lib/plexDetail; the cells of the channel columns
  and the registry's page wrappers are components files of their own (channelCells, modulePages).
- usePersistedTab, an alias of useAccountPersistedState, is gone; its callers use the original.

npm run lint is now eslint --max-warnings 0, so a warning fails the gate like an error does.

frontend/src/components/AdminUsers.tsx

@@ -35,8 +35,9 @@ import SegmentedToggle from "./ui/SegmentedToggle";
 import { LANGUAGES, type LangCode } from "../i18n";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 import AccountPanel from "./AccountPanel";
-import { useConfirm } from "./ConfirmProvider";
-import Tabs, { usePersistedTab } from "./ui/Tabs";
+import { useConfirm } from "./confirmContext";
+import Tabs from "./ui/Tabs";
+import { useAccountPersistedState } from "../lib/storage";
 import { PageToolbar } from "./ui/PageShell";
 import { useMe } from "../lib/useMe";
 import { notifyActionError, notifyKnownFailure } from "../lib/actionErrors";
@@ -50,7 +51,7 @@ import { notifyActionError, notifyKnownFailure } from "../lib/actionErrors";
 export default function AdminUsers() {
   const me = useMe();
   const { t } = useTranslation();
-  const [tab, setTab] = usePersistedTab(ADMIN_USERS_TAB_KEY, "roles");
+  const [tab, setTab] = useAccountPersistedState(ADMIN_USERS_TAB_KEY, "roles");
   const tabs = [
     { id: "roles", label: t("users.tabs.roles") },
     // Both kinds of "waiting on the admin": a request needing a decision, and a sign-up whose