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/wizardContext.ts

@@ -0,0 +1,20 @@
+import { createContext, useContext } from "react";
+
+// The onboarding wizard's actions context + hook, split out of WizardProvider so that file exports
+// only components (Fast Refresh).
+
+export interface WizardActions {
+  openWizard: () => void;
+  closeWizard: () => void;
+}
+
+// Only the actions are shared — the open flag stays local to the provider (it renders the wizard
+// itself), so no consumer needs to subscribe to it.
+export const WizardActionsContext = createContext<WizardActions>({
+  openWizard: () => {},
+  closeWizard: () => {},
+});
+
+export function useWizard(): WizardActions {
+  return useContext(WizardActionsContext);
+}