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/Playlists.dom.test.tsx

@@ -34,8 +34,8 @@ vi.mock("../lib/notifications", async (importOriginal) => ({
   ...(await importOriginal<typeof import("../lib/notifications")>()),
   notify: (...a: unknown[]) => h.notify(...a),
 }));
-vi.mock("./PlaylistsProvider", () => ({ usePlaylists: () => ({ selectedId: h.selectedId }) }));
-vi.mock("./ConfirmProvider", () => ({ useConfirm: () => async () => true }));
+vi.mock("./playlistsContext", () => ({ usePlaylists: () => ({ selectedId: h.selectedId }) }));
+vi.mock("./confirmContext", () => ({ useConfirm: () => async () => true }));
 vi.mock("./usePlaylistActions", () => ({ usePlaylistActions: () => ({ remove: vi.fn() }) }));
 vi.mock("./PlayerModal", async () => {
   const { useEffect } = await import("react");