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/lib/adminUsersTab.ts

@@ -2,7 +2,7 @@ import { LS, setAccountRaw } from "./storage";
 
 // The admin Users page's persisted-tab key + the Access-requests tab id, plus a helper to
 // pre-select that tab before navigating there (a notification's "Review" link uses it —
-// usePersistedTab reads the key on mount, and the page mounts fresh on navigation).
+// useAccountPersistedState reads the key on mount, and the page mounts fresh on navigation).
 //
 // Kept out of AdminUsers.tsx so callers (App, NotificationsPanel) can pre-select the tab
 // WITHOUT statically importing the — now lazy-loaded — admin page, which would otherwise pull