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/ui/PageShell.tsx

@@ -1,6 +1,5 @@
 import {
   cloneElement,
-  createContext,
   isValidElement,
   useContext,
   useLayoutEffect,
@@ -11,12 +10,7 @@ import {
 } from "react";
 import { createPortal } from "react-dom";
 import PageScroller from "./PageScroller";
-
-// The DOM node a module's toolbar portals into (see PageToolbar below). PageShell provides its
-// fixed band as the default target; a module can override it with its own <PageToolbarSlot.Provider>
-// to redirect its toolbar somewhere else — the channel page points it at a sticky sub-header so the
-// toolbar rides with the tabs instead of the shell's top band.
-export const PageToolbarSlot = createContext<HTMLElement | null>(null);
+import { PageToolbarSlot } from "./pageToolbarSlot";
 
 // A module renders its per-module fixed band (filter chips / sort / counts) through this. The
 // content lands in the shell's toolbar band — ABOVE the scroller, so it stays put while the content