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/NavSidebar.tsx

@@ -2,7 +2,7 @@ import { useState, useSyncExternalStore } from "react";
 import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery } from "@tanstack/react-query";
-import { useNavigation, useNavigationActions } from "./NavigationProvider";
+import { useNavigation, useNavigationActions } from "./navigationContext";
 import { Info, LogOut, Settings, Shield, UserPlus, type LucideIcon } from "lucide-react";
 import { api, type Me } from "../lib/api";
 import { pinOnNextPage } from "../lib/activeAccount";
@@ -17,7 +17,8 @@ import { Button } from "./ui/Button";
 import AvatarImg from "./ui/Avatar";
 import Tooltip from "./ui/Tooltip";
 import LanguageSwitcher from "./LanguageSwitcher";
-import { PopoverSurface, useAnchoredPopover } from "./ui/AnchoredPopover";
+import { PopoverSurface } from "./ui/AnchoredPopover";
+import { useAnchoredPopover } from "./ui/anchoredPopoverHooks";
 
 // Primary app navigation: a thin icon-only left rail. The modules used to hide under the avatar
 // dropdown; they now live here, each revealing its name in a hover/focus flyout label. Account