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/PersonalViewsModal.dom.test.tsx

@@ -3,7 +3,7 @@
 // S7 — the Personal-views manager gains an EDITABLE detail pane: captured filters are now removable
 // chips that write the pruned set back to the stored view (undoable), an explicit Save-current button
 // stores the live filters into the view, and rename has a visible commit affordance. This drives the
-// real modal against a mocked SavedViewsProvider so we can assert exactly which mutation fires; the
+// real modal against a mocked savedViewsContext so we can assert exactly which mutation fires; the
 // captured-filter row uses the REAL useActiveFilters (fed a tag from a seeded react-query cache), so
 // the prune snapshots are the genuine ones the live chip row computes.
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
@@ -56,7 +56,7 @@ const CTX = {
   toggleDefault: vi.fn(async () => {}),
 };
 
-vi.mock("./SavedViewsProvider", () => ({
+vi.mock("./savedViewsContext", () => ({
   useSavedViews: () => ({
     ...CTX,
     activeViewId: h.activeViewId,