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/PrefsProvider.dom.test.tsx

@@ -7,7 +7,8 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { act, cleanup, render } from "@testing-library/react";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
-import { PrefsProvider, usePrefs } from "./PrefsProvider";
+import { PrefsProvider } from "./PrefsProvider";
+import { usePrefs } from "./prefsContext";
 import { api } from "../lib/api";
 import { ACTIVE_ACCOUNT_KEY } from "../lib/activeAccount";
 import { accountKey, LS } from "../lib/storage";