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/Sidebar.tsx

@@ -9,7 +9,7 @@ import { useDebounced } from "../lib/useDebounced";
 import { useActiveFilters } from "../lib/useActiveFilters";
 import TagManager from "./TagManager";
 import SavedViewsPopover from "./SavedViewsPopover";
-import { useSavedViews } from "./SavedViewsProvider";
+import { useSavedViews } from "./savedViewsContext";
 import FilterRail from "./FilterRail";
 import { type PanelItem } from "./ui/PanelGroups";
 import { type MediaSourceKind } from "../lib/mediaSource";
@@ -17,8 +17,8 @@ import ChannelPicker from "./ChannelPicker";
 import DateFilterSelect from "./DateFilterSelect";
 import YearFilter from "./YearFilter";
 import Combobox, { type ComboboxOption } from "./ui/Combobox";
-import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
-import { useChannelsActions } from "./ChannelsProvider";
+import { useFeedFilters, useFeedFiltersActions } from "./feedFiltersContext";
+import { useChannelsActions } from "./channelsContext";
 import { useMe } from "../lib/useMe";
 
 // Filter ids; display labels resolved at render time via t("sidebar.sort.<id>") etc.