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/useContextMenu.ts

@@ -1,7 +1,7 @@
 import { useCallback, useRef, useState, type PointerEvent as ReactPointerEvent } from "react";
 
 import { matchesShortcut, useKeyScope } from "../../lib/keymap";
-import { useAnchoredPopover } from "./AnchoredPopover";
+import { useAnchoredPopover } from "./anchoredPopoverHooks";
 
 // E10 S2 — the right-click context menu's INPUT half: the gestures that open it and the row they
 // belong to. The surface itself is `<ContextMenu>` (ContextMenu.tsx); they are two files only so