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/PlaylistsProvider.tsx

@@ -1,8 +1,14 @@
-import { createContext, useCallback, useContext, useMemo, useState, type ReactNode } from "react";
+import { useCallback, useMemo, useState, type ReactNode } from "react";
 import { LS, setAccountRaw } from "../lib/storage";
 import { useUrlMirror } from "../lib/useUrlMirror";
-import { useNavigation } from "./NavigationProvider";
+import { useNavigation } from "./navigationContext";
 import { readSelectedId } from "./playlistsInit";
+import {
+  PlaylistsActionsContext,
+  PlaylistsStateContext,
+  type PlaylistsActions,
+  type PlaylistsState,
+} from "./playlistsContext";
 
 // Owns the Playlists module's shared local state — the rail's name filter and the selected playlist
 // — lifted out of the App god component so the header search box, the PlaylistsRail, and the
@@ -14,29 +20,8 @@ import { readSelectedId } from "./playlistsInit";
 // playlist is per-account persisted (LS.playlist) so a reload returns you to it. Neither is
 // server-synced, and an account switch does a full reload, so init-from-storage is all the
 // hydration these need — no per-account load effect (unlike the server-backed feed filters).
-
-type PlaylistsState = {
-  search: string;
-  selectedId: number | null;
-};
-
-interface PlaylistsActions {
-  setSearch: (q: string) => void;
-  setSelectedId: (id: number | null) => void;
-}
-
-const StateContext = createContext<PlaylistsState>({ search: "", selectedId: null });
-const ActionsContext = createContext<PlaylistsActions>({
-  setSearch: () => {},
-  setSelectedId: () => {},
-});
-
-export function usePlaylists(): PlaylistsState {
-  return useContext(StateContext);
-}
-export function usePlaylistsActions(): PlaylistsActions {
-  return useContext(ActionsContext);
-}
+//
+// The contexts + hooks live in playlistsContext.ts.
 
 export function PlaylistsProvider({ children }: { children: ReactNode }) {
   const [search, setSearchState] = useState("");
@@ -61,8 +46,8 @@ export function PlaylistsProvider({ children }: { children: ReactNode }) {
   );
 
   return (
-    <ActionsContext.Provider value={actions}>
-      <StateContext.Provider value={state}>{children}</StateContext.Provider>
-    </ActionsContext.Provider>
+    <PlaylistsActionsContext.Provider value={actions}>
+      <PlaylistsStateContext.Provider value={state}>{children}</PlaylistsStateContext.Provider>
+    </PlaylistsActionsContext.Provider>
   );
 }