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/Home.dom.test.tsx

@@ -58,10 +58,10 @@ vi.mock("../lib/api", () => ({
 // WHAT is handed over is: the jump target decides both what plays and where Back goes.
 const setPage = vi.fn();
 const setJumpTo = vi.fn();
-vi.mock("./NavigationProvider", () => ({
+vi.mock("./navigationContext", () => ({
   useNavigationActions: () => ({ setPage: (p: string) => setPage(p) }),
 }));
-vi.mock("./PlexProvider", () => ({
+vi.mock("./plexContext", () => ({
   usePlexActions: () => ({ setJumpTo: (t: unknown) => setJumpTo(t) }),
 }));