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/NavigationProvider.tsx

@@ -1,13 +1,4 @@
-import {
-  createContext,
-  useCallback,
-  useContext,
-  useEffect,
-  useMemo,
-  useRef,
-  useState,
-  type ReactNode,
-} from "react";
+import { useCallback, useEffect, useMemo, useRef, useState, type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
 import { urlWithParams, type Page } from "../lib/urlState";
 import { pageTitleKey } from "../lib/pageMeta";
@@ -20,57 +11,19 @@ import {
   rememberNavPosition,
 } from "../lib/history";
 import { channelViewFrom, loadInitialPage, PAGE_KEY, type ChannelView } from "./navigationInit";
+import {
+  NavigationActionsContext,
+  NavigationStateContext,
+  type NavigationActions,
+  type NavigationState,
+} from "./navigationContext";
 
 // Owns the app's navigation + browser-history state, lifted out of the App god component so pages
 // consume it via a hook instead of prop-drilling `page`/`setPage`/`onOpenChannel` through the tree.
 // Mirrors ConfirmProvider (context + default value + one-line hooks); composed in main.tsx inside
 // ConfirmProvider, around <App/>.
 //
-// TWO contexts on purpose: a STABLE actions context (setPage/openChannel/… never change reference)
-// and a CHANGING state context (page/channelView/ytSearch). Context consumers re-render whenever the
-// value changes, so deep memo'd leaves (VideoCard, PlayerModal) subscribe to actions ONLY and never
-// re-render on a page/channel switch — preserving the memo that keeps feed appends from re-rendering
-// every existing card.
-
-export interface NavigationState {
-  page: Page;
-  /** The open channel page (null = none). Overlays `page`; rides in history.state._chan. */
-  channelView: ChannelView | null;
-  /** Live YouTube search term (null = normal feed). A feed sub-view; rides in history.state._yt. */
-  ytSearch: string | null;
-}
-
-export interface NavigationActions {
-  /** Switch modules. Closes every open overlay first and pushes only once their history entries are
-   *  back (`lib/history.closeOverlaysThenNavigate`), so over an open player the push lands a moment
-   *  after the call; with nothing open it is synchronous. The requested page being already current
-   *  (with no channel page over it) pushes nothing — but the overlays still close. */
-  setPage: (next: Page) => void;
-  openChannel: (id: string, name?: string) => void;
-  closeChannel: () => void;
-  enterYtSearch: (q: string) => void;
-  exitYtSearch: () => void;
-}
-
-const StateContext = createContext<NavigationState>({
-  page: "feed",
-  channelView: null,
-  ytSearch: null,
-});
-const ActionsContext = createContext<NavigationActions>({
-  setPage: () => {},
-  openChannel: () => {},
-  closeChannel: () => {},
-  enterYtSearch: () => {},
-  exitYtSearch: () => {},
-});
-
-export function useNavigation(): NavigationState {
-  return useContext(StateContext);
-}
-export function useNavigationActions(): NavigationActions {
-  return useContext(ActionsContext);
-}
+// The contexts, their value types and the hooks live in navigationContext.ts.
 
 export function NavigationProvider({ children }: { children: ReactNode }) {
   const { t, i18n } = useTranslation();
@@ -256,8 +209,8 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
   );
 
   return (
-    <ActionsContext.Provider value={actions}>
-      <StateContext.Provider value={state}>{children}</StateContext.Provider>
-    </ActionsContext.Provider>
+    <NavigationActionsContext.Provider value={actions}>
+      <NavigationStateContext.Provider value={state}>{children}</NavigationStateContext.Provider>
+    </NavigationActionsContext.Provider>
   );
 }