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/tableHeader.tsx

@@ -1,10 +1,12 @@
 import { useRef, useState, type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
 import { ArrowDown, ArrowUp, ListFilter, X } from "lucide-react";
-import { PopoverItem, PopoverSurface, useAnchoredPopover } from "./AnchoredPopover";
+import { PopoverItem, PopoverSurface } from "./AnchoredPopover";
+import { useAnchoredPopover } from "./anchoredPopoverHooks";
 import { useScrollFade } from "../../lib/useScrollFade";
 import { type SortState } from "../../lib/columnSort";
 import { isActive, type Column, type FilterMap } from "../../lib/tableColumns";
+import { alignClass } from "./tableAlign";
 
 // The column-header machinery — the sortable header row with its filter funnel + popover — lifted
 // OUT of DataTable so a non-DataTable body can render under the same headers. The column contract
@@ -17,9 +19,6 @@ import { isActive, type Column, type FilterMap } from "../../lib/tableColumns";
 // playlist keep its stored order and still get real column headers. Same move as E4 S3's
 // ChannelLayoutGrid, which builds a card view straight from the `Column<T>` render closures.
 
-export const alignClass = (a?: "left" | "right" | "center") =>
-  a === "right" ? "text-right" : a === "center" ? "text-center" : "text-left";
-
 // The multi-select filter's option list, capped and scrollable. Its own component because it owns a
 // hook — see the FilterPopover note below for why nesting either of these would remount them.
 function FilterOptionList({ children }: { children: ReactNode }) {