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/api/tagModes.test.ts

@@ -1,7 +1,7 @@
 import { describe, it, expect } from "vitest";
 import { coerceTagModes, coerceUntagged, DEFAULT_TAG_MODES } from "./tagModes";
 
-// coerceTagModes runs on every persisted saved-view blob (SavedViewsProvider.asFilters), on the URL
+// coerceTagModes runs on every persisted saved-view blob (savedViewsContext.asFilters), on the URL
 // parse, and on the localStorage seed — the single migration point for the legacy global `tagMode`.
 describe("coerceTagModes", () => {
   it("defaults to all-'or' for empty/undefined input", () => {