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/PlexCollectionView.tsx

@@ -12,7 +12,7 @@ import { healPlexSort, plexSortOptions } from "../lib/plexSort";
 import { usePref } from "../lib/usePref";
 import SortControl from "./ui/SortControl";
 import { type SortState } from "../lib/columnSort";
-import { useConfirm } from "./ConfirmProvider";
+import { useConfirm } from "./confirmContext";
 import { BackBtn, ActionBtn, DetailHero } from "../lib/plexDetailUi";
 import { PlexPosterCard } from "./plexCards";
 import { PlexPinMenu } from "./ComfortPinMenu";