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/PlexProvider.tsx

@@ -1,12 +1,4 @@
-import {
-  createContext,
-  useCallback,
-  useContext,
-  useEffect,
-  useMemo,
-  useState,
-  type ReactNode,
-} from "react";
+import { useCallback, useEffect, useMemo, useState, type ReactNode } from "react";
 import { coercePlexFilters, EMPTY_PLEX_FILTERS, type PlexFilters } from "../lib/api";
 import { parseSortState, type SortState } from "../lib/columnSort";
 import { parsePlexField, type PlexSearchField } from "../lib/plexSearch";
@@ -20,7 +12,14 @@ import {
 import { getAccountRaw, LS, setAccountRaw } from "../lib/storage";
 import { logFilterChange } from "../lib/filterLog";
 import { useUrlMirror } from "../lib/useUrlMirror";
-import { useNavigation } from "./NavigationProvider";
+import { useNavigation } from "./navigationContext";
+import {
+  PlexActionsContext,
+  PlexStateContext,
+  type PlexActions,
+  type PlexJumpTarget,
+  type PlexState,
+} from "./plexContext";
 
 // Owns the Plex module's shared local state — the left-rail library scope / watch-state / sort /
 // expanded filters, the header search term, and the "open this playlist" hand-off — lifted out of
@@ -33,73 +32,8 @@ import { useNavigation } from "./NavigationProvider";
 // hand-off (the rail sets it, PlexBrowse consumes it into a history subview then clears it). The Plex
 // search term is deliberately its OWN state, NOT the feed's `filters.q`, so a Plex search never leaks
 // into (and is never greeted by a stale value from) the feed.
-
-// A "jump straight to this title" hand-off: PlexBrowse opens a movie's info page (like the card's ⓘ),
-// a show's drill-down (like clicking a show card), or — for E24's Home hero, whose whole promise is a
-// single Play button — the PLAYER for a movie/episode rating_key. `play` exists because the player
-// lives inside this module with its lazy hls.js and its provider tree; mounting a second one on the
-// landing page to avoid one hand-off would be the expensive way round.
-export type PlexJumpTarget = {
-  id: string;
-  type: "movie" | "show" | "play";
-  /** The jump ARRIVED WITH a page change — another module set this and sent the user here in one
-   *  gesture (the Home surface plays a Plex title). The opened sub-view then REPLACES the page entry
-   *  that hand-off just pushed, instead of stacking on top of it, so Back returns to where the user
-   *  actually came from rather than to a module root they never visited. Omitted for a jump made
-   *  from INSIDE the Plex module (the header typeahead), where the entry being replaced would be a
-   *  sub-view the user really was looking at. */
-  replace?: boolean;
-};
-
-type PlexState = {
-  scope: string; // movie | show | both (unified cross-library scope)
-  show: string;
-  sort: SortState; // {key,dir}; healed against the scope's options at the point of use
-  filters: PlexFilters;
-  q: string;
-  field: PlexSearchField; // search scope-pill; healed against `scope` at the point of use
-  playlistOpen: number | null;
-  jumpTo: PlexJumpTarget | null; // typeahead → grid hand-off ("open this title")
-};
-
-interface PlexActions {
-  setScope: (v: string) => void;
-  setShow: (v: string) => void;
-  setSort: (v: SortState) => void;
-  setFilters: (f: PlexFilters) => void;
-  setQ: (q: string) => void;
-  setField: (f: PlexSearchField) => void;
-  setPlaylistOpen: (id: number | null) => void;
-  setJumpTo: (t: PlexJumpTarget | null) => void;
-}
-
-const StateContext = createContext<PlexState>({
-  scope: "both",
-  show: "all",
-  sort: PLEX_SORT_FALLBACK,
-  filters: EMPTY_PLEX_FILTERS,
-  q: "",
-  field: "all",
-  playlistOpen: null,
-  jumpTo: null,
-});
-const ActionsContext = createContext<PlexActions>({
-  setScope: () => {},
-  setShow: () => {},
-  setSort: () => {},
-  setFilters: () => {},
-  setQ: () => {},
-  setField: () => {},
-  setPlaylistOpen: () => {},
-  setJumpTo: () => {},
-});
-
-export function usePlex(): PlexState {
-  return useContext(StateContext);
-}
-export function usePlexActions(): PlexActions {
-  return useContext(ActionsContext);
-}
+//
+// The contexts, their value types and the hooks live in plexContext.ts.
 
 // The per-account init readers, shared by the useState seeds and the shared-link seed's base.
 function readPlexScope(): string {
@@ -268,8 +202,8 @@ export function PlexProvider({ children }: { children: ReactNode }) {
   useUrlMirror(plexParamUpdates(page === "plex" ? { scope, sort, show, filters } : null));
 
   return (
-    <ActionsContext.Provider value={actions}>
-      <StateContext.Provider value={state}>{children}</StateContext.Provider>
-    </ActionsContext.Provider>
+    <PlexActionsContext.Provider value={actions}>
+      <PlexStateContext.Provider value={state}>{children}</PlexStateContext.Provider>
+    </PlexActionsContext.Provider>
   );
 }