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/AddToPlaylist.tsx

@@ -7,7 +7,8 @@ import { api, type Playlist } from "../lib/api";
 import { notifyActionError } from "../lib/actionErrors";
 import { useScrollFade } from "../lib/useScrollFade";
 import { playlistName } from "../lib/playlistName";
-import { PopoverItem, PopoverSurface, useAnchoredPopover } from "./ui/AnchoredPopover";
+import { PopoverItem, PopoverSurface } from "./ui/AnchoredPopover";
+import { useAnchoredPopover } from "./ui/anchoredPopoverHooks";
 
 // A small anchored menu for toggling a video's membership in the user's playlists, with inline
 // "new playlist" creation. Same shape as the channel tag editor (R8 S1's pilot): radio-style