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/SettingsPanel.tsx

@@ -15,11 +15,11 @@ import Tooltip from "./ui/Tooltip";
 import { inputCls, Section, SettingRow, Switch } from "./ui/form";
 import { Button } from "./ui/Button";
 import { type SaveState } from "./ui/DraftSaveBar";
-import { useConfirm } from "./ConfirmProvider";
+import { useConfirm } from "./confirmContext";
 import { StateMessage } from "./ui/QueryState";
-import { useWizard } from "./WizardProvider";
+import { useWizard } from "./wizardContext";
 import { START_ON_HOME } from "./navigationInit";
-import { usePrefs } from "./PrefsProvider";
+import { usePrefs } from "./prefsContext";
 import Tabs from "./ui/Tabs";
 import { useMe } from "../lib/useMe";