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/SavedViewsProvider.dom.test.tsx

@@ -20,8 +20,8 @@ vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
   return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
 });
-vi.mock("./NavigationProvider", () => ({ useNavigation: () => ({ page: h.page }) }));
-vi.mock("./FeedFiltersProvider", () => ({
+vi.mock("./navigationContext", () => ({ useNavigation: () => ({ page: h.page }) }));
+vi.mock("./feedFiltersContext", () => ({
   useFeedFilters: () => ({
     tags: [],
     channelIds: [],
@@ -36,13 +36,14 @@ vi.mock("./FeedFiltersProvider", () => ({
   }),
   useFeedFiltersActions: () => ({ setFilters: h.setFeedFilters }),
 }));
-vi.mock("./PlexProvider", () => ({
+vi.mock("./plexContext", () => ({
   usePlex: () => ({ filters: {} }),
   usePlexActions: () => ({ setFilters: h.setPlexFilters }),
 }));
-vi.mock("./ConfirmProvider", () => ({ useConfirm: () => async () => true }));
+vi.mock("./confirmContext", () => ({ useConfirm: () => async () => true }));
 
-import { SavedViewsProvider, useSavedViews } from "./SavedViewsProvider";
+import { SavedViewsProvider } from "./SavedViewsProvider";
+import { useSavedViews } from "./savedViewsContext";
 import { api, type SavedView } from "../lib/api";
 import { LS, accountKey } from "../lib/storage";
 import { ACTIVE_ACCOUNT_KEY } from "../lib/activeAccount";