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/PlayerModal.tsx

@@ -1,7 +1,7 @@
 import { useEffect, useId, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { createPortal } from "react-dom";
-import { useNavigationActions } from "./NavigationProvider";
+import { useNavigationActions } from "./navigationContext";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
 import { usePref } from "../lib/usePref";
@@ -26,7 +26,8 @@ import {
 } from "lucide-react";
 import Avatar from "./ui/Avatar";
 import AddToPlaylist from "./AddToPlaylist";
-import { PopoverSurface, useAnchoredPopover } from "./ui/AnchoredPopover";
+import { PopoverSurface } from "./ui/AnchoredPopover";
+import { useAnchoredPopover } from "./ui/anchoredPopoverHooks";
 import {
   pushLayer,
   removeLayer,
@@ -50,7 +51,8 @@ import { fullscreenElement } from "../lib/fullscreen";
 import { dispatch, isActivationTarget, useKeyScope } from "../lib/keymap";
 import { useBackToClose } from "../lib/history";
 import ChannelNameLink from "./ChannelNameLink";
-import Toaster, { useOverlayOrder } from "./ui/Toaster";
+import Toaster from "./ui/Toaster";
+import { useOverlayOrder } from "./ui/overlayOrder";
 import { afterRemoval } from "../lib/playerQueue";
 import {
   useNativeControlsYield,