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/Playlists.tsx

@@ -58,13 +58,14 @@ import { useContextMenu } from "./ui/useContextMenu";
 import UndoToolbar from "./ui/UndoToolbar";
 import NavGrid from "./ui/NavGrid";
 import { PageToolbar } from "./ui/PageShell";
-import { useConfirm } from "./ConfirmProvider";
+import { useConfirm } from "./confirmContext";
 import { usePlaylistActions } from "./usePlaylistActions";
-import { usePlaylists } from "./PlaylistsProvider";
+import { usePlaylists } from "./playlistsContext";
 import { useMe } from "../lib/useMe";
 import { usePref } from "../lib/usePref";
 import { navItemAttrs } from "../lib/listNav";
-import { ColumnHead, alignClass } from "./ui/tableHeader";
+import { ColumnHead } from "./ui/tableHeader";
+import { alignClass } from "./ui/tableAlign";
 import { filterRows, isActive, type Column, type FilterMap } from "../lib/tableColumns";
 import { GRIP_KEY, playlistColumns } from "./playlistColumns";