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/PlexInfo.tsx

@@ -6,20 +6,14 @@ import { Check, ExternalLink, FolderPlus, ListPlus, Play, RotateCcw, Star, X } f
 import { api, type PlexFilters, type PlexItemDetail } from "../lib/api";
 import { formatRuntime } from "../lib/format";
 import { notifyActionError } from "../lib/actionErrors";
-import {
-  DetailCustomizeMenu,
-  Filterable,
-  PrefToggle,
-  tmdbBackdropUrl,
-  useCrossfadeBackdrop,
-  useDetailPrefs,
-} from "../lib/plexDetailUi";
+import { DetailCustomizeMenu, Filterable, PrefToggle } from "../lib/plexDetailUi";
+import { tmdbBackdropUrl, useCrossfadeBackdrop, useDetailPrefs } from "../lib/plexDetail";
 import { usePref } from "../lib/usePref";
 import i18n from "../i18n";
 import { Carousel } from "./ui/Carousel";
 import PlexCollectionEditor from "./PlexCollectionEditor";
 import PlexPlaylistAdd from "./PlexPlaylistAdd";
-import { PLAYABLE_TINT, playableClass } from "./plexCards";
+import { PLAYABLE_TINT, playableClass } from "./plexPlayable";
 import { navItemAttrs } from "../lib/listNav";
 
 // The rich "media info" surface for a Plex item — poster/art, IMDb score + link, content rating,