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/FeedViewProvider.tsx

@@ -1,7 +1,12 @@
-import { createContext, useContext, useMemo, type ReactNode } from "react";
+import { useMemo, type ReactNode } from "react";
 import { parseFeedView, type FeedView } from "../lib/feedView";
 import { LS } from "../lib/storage";
 import { usePref } from "../lib/usePref";
+import {
+  FeedViewActionsContext,
+  FeedViewStateContext,
+  type FeedViewActions,
+} from "./feedViewContext";
 
 // Owns the feed's view mode (the card/row/tile density switcher), lifted out of App so both the feed
 // page and the channel page read it via hooks instead of prop-drilling `view`/`setView` through App.
@@ -10,20 +15,8 @@ import { usePref } from "../lib/usePref";
 // The view mode is an account PREFERENCE, not a filter: it persists to the server on pick and follows
 // the account across devices. usePref carries that server-sync (seeded from the server pref, with the
 // localStorage mirror as the fallback) so this provider is just the two contexts over it.
-
-interface FeedViewActions {
-  setView: (v: FeedView) => void;
-}
-
-const StateContext = createContext<FeedView>("cards");
-const ActionsContext = createContext<FeedViewActions>({ setView: () => {} });
-
-export function useFeedView(): FeedView {
-  return useContext(StateContext);
-}
-export function useFeedViewActions(): FeedViewActions {
-  return useContext(ActionsContext);
-}
+//
+// The contexts + hooks live in feedViewContext.ts.
 
 export function FeedViewProvider({ children }: { children: ReactNode }) {
   // parseFeedView also maps the pre-0.45 "grid"/"list" values forward (idempotent for current values).
@@ -35,8 +28,8 @@ export function FeedViewProvider({ children }: { children: ReactNode }) {
   const actions = useMemo<FeedViewActions>(() => ({ setView }), [setView]);
 
   return (
-    <ActionsContext.Provider value={actions}>
-      <StateContext.Provider value={view}>{children}</StateContext.Provider>
-    </ActionsContext.Provider>
+    <FeedViewActionsContext.Provider value={actions}>
+      <FeedViewStateContext.Provider value={view}>{children}</FeedViewStateContext.Provider>
+    </FeedViewActionsContext.Provider>
   );
 }