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/adminInvites.dom.test.tsx

@@ -31,9 +31,8 @@ type ConfirmOpts = {
   danger?: boolean;
 };
 const confirmMock = vi.fn(async (_opts: ConfirmOpts) => true);
-vi.mock("./ConfirmProvider", () => ({
+vi.mock("./confirmContext", () => ({
   useConfirm: () => confirmMock,
-  ConfirmProvider: ({ children }: { children: React.ReactNode }) => children,
 }));
 
 const notifyMock = vi.fn();
@@ -53,9 +52,10 @@ vi.mock("../lib/useMe", () => ({ useMe: () => meMock }));
 
 // The tab strip is rendered into an app-level toolbar slot that does not exist here, so pin the
 // active tab directly — this file is about the Access-requests panel, not the tab chrome.
-vi.mock("./ui/Tabs", () => ({
-  default: () => null,
-  usePersistedTab: () => ["access", () => {}],
+vi.mock("./ui/Tabs", () => ({ default: () => null }));
+vi.mock("../lib/storage", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("../lib/storage")>()),
+  useAccountPersistedState: () => ["access", () => {}],
 }));
 
 const listMock = vi.fn();