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/WriteFailureReporting.dom.test.tsx

@@ -17,7 +17,7 @@ vi.mock("react-i18next", async (importOriginal) => {
     useTranslation: () => ({ t: (key: string) => key, i18n: { language: "en" } }),
   };
 });
-vi.mock("./ConfirmProvider", () => ({ useConfirm: () => async () => true }));
+vi.mock("./confirmContext", () => ({ useConfirm: () => async () => true }));
 const { notify } = vi.hoisted(() => ({
   notify: vi.fn((_n: { level: string; title?: string; message: string }) => 0),
 }));