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/ChannelPage.tsx

@@ -4,7 +4,7 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { channelDetailPollMs, useBackfillFeedRefresh } from "../lib/channelDetailPoll";
 import { qk } from "../lib/queryKeys";
-import { useNavigationActions } from "./NavigationProvider";
+import { useNavigationActions } from "./navigationContext";
 import { PriorityStepper } from "./ui/PriorityStepper";
 import { useChannelPriority } from "../lib/useChannelPriority";
 import {
@@ -24,9 +24,10 @@ import Avatar from "./ui/Avatar";
 import Tabs from "./ui/Tabs";
 import Feed from "./Feed";
 import ChannelAboutContent from "./ChannelAbout";
-import { PageToolbar, PageToolbarSlot } from "./ui/PageShell";
-import { useFeedFilters } from "./FeedFiltersProvider";
-import { useConfirm } from "./ConfirmProvider";
+import { PageToolbar } from "./ui/PageShell";
+import { PageToolbarSlot } from "./ui/pageToolbarSlot";
+import { useFeedFilters } from "./feedFiltersContext";
+import { useConfirm } from "./confirmContext";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { notifyActionError } from "../lib/actionErrors";