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.tsx

@@ -1,7 +1,5 @@
 import {
-  createContext,
   useCallback,
-  useContext,
   useEffect,
   useLayoutEffect,
   useMemo,
@@ -23,13 +21,14 @@ import { configureNotifications, getNotifSettings, type NotifSettings } from "..
 import { hintsEnabled, setHintsEnabled } from "../lib/hints";
 import { getAccountRaw, LS, setAccountRaw } from "../lib/storage";
 import type { PrefsController } from "./SettingsPanel";
+import { PrefsContext } from "./prefsContext";
 
 const PERF_KEY = LS.perfMode;
 
 // The four Settings-page preferences. They apply locally for instant preview and auto-save (debounced)
 // to the server — there is no draft or Save button. This provider owns them (lifted out of the App god
 // component) plus the last-saved "baseline" (the server values it was seeded with, then each successful
-// save) used to suppress saving what the server already holds. Consumers (SettingsPanel, GlassTuner) read the PrefsController via usePrefs().
+// save) used to suppress saving what the server already holds. Consumers (SettingsPanel, GlassTuner) read the PrefsController via usePrefs() (prefsContext.ts).
 type EditablePrefs = {
   theme: ThemePrefs;
   performanceMode: boolean;
@@ -37,22 +36,6 @@ type EditablePrefs = {
   notifications: NotifSettings;
 };
 
-const PrefsContext = createContext<PrefsController>({
-  theme: DEFAULT_THEME,
-  setTheme: () => {},
-  perf: false,
-  setPerf: () => {},
-  hints: true,
-  setHints: () => {},
-  notif: getNotifSettings(),
-  setNotif: () => {},
-  saveState: "idle",
-});
-
-export function usePrefs(): PrefsController {
-  return useContext(PrefsContext);
-}
-
 export function PrefsProvider({ children }: { children: ReactNode }) {
   const qc = useQueryClient();
   // The account's prefs, seeded straight from the `me` cache — BootGate mounts this only once /api/me