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/ui/Tabs.tsx

@@ -1,5 +1,4 @@
 import { type LucideIcon } from "lucide-react";
-import { useAccountPersistedState } from "../../lib/storage";
 import CountBadge from "./CountBadge";
 
 // Reusable pill tab-rail for in-page sub-navigation (Configuration, Users & roles, the Settings
@@ -7,8 +6,8 @@ import CountBadge from "./CountBadge";
 // the Settings-style side rail (full-width, left-aligned, icon + label) — from the `sm` breakpoint up.
 // Below it a side rail leaves a phone too little width for the content beside it (measured at 390 px:
 // the rail clipped its own labels and Settings → Account pushed its buttons over their text), so there
-// it is a horizontal row that scrolls. The active tab is persisted to localStorage so a reload keeps
-// the user where they were.
+// it is a horizontal row that scrolls. Callers persist the active tab with lib/storage's
+// useAccountPersistedState (account-scoped), so a reload keeps the user where they were.
 
 export interface TabDef {
   id: string;
@@ -18,10 +17,6 @@ export interface TabDef {
   testId?: string; // optional data-testid, to preserve existing e2e hooks
 }
 
-/** Persisted active-tab state, scoped to the current account (so one account's last tab doesn't
- * carry into another in the same browser) — re-exported under its original name for call sites. */
-export const usePersistedTab = useAccountPersistedState;
-
 export default function Tabs({
   tabs,
   active,