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/ConfirmProvider.tsx

@@ -1,65 +1,10 @@
-import { createContext, useCallback, useContext, useState, type ReactNode } from "react";
+import { useCallback, useState, type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
 import Modal from "./ui/Modal";
 import { Button } from "./ui/Button";
+import { ChoiceContext, type ChoiceFn, type ChoiceOptions } from "./confirmContext";
 
-// App-styled, promise-based replacement for window.confirm. Wrap the tree in
-// <ConfirmProvider> once, then anywhere:  const confirm = useConfirm();
-//   if (await confirm({ message: "…", danger: true })) { … }
-//
-// For a decision with more than two outcomes use `useChoice()` instead — never chain two
-// confirms, because the second dialog's Cancel/Escape/backdrop is then indistinguishable from
-// a deliberate answer (the playlist delete used to read a dismissal as "delete here only" and
-// went ahead and deleted).
-interface ConfirmOptions {
-  title?: string;
-  message: ReactNode;
-  confirmLabel?: string;
-  cancelLabel?: string;
-  danger?: boolean;
-}
-
-interface Choice {
-  id: string;
-  label: string;
-  danger?: boolean;
-}
-
-export interface ChoiceOptions {
-  title?: string;
-  message: ReactNode;
-  cancelLabel?: string;
-  /** Rendered left→right after Cancel. Resolves to the clicked choice's `id`. */
-  choices: Choice[];
-}
-
-type ChoiceFn = (opts: ChoiceOptions) => Promise<string | null>;
-type ConfirmFn = (opts: ConfirmOptions) => Promise<boolean>;
-
-const ChoiceContext = createContext<ChoiceFn>(() => Promise.resolve(null));
-
-/** Two outcomes: resolves true on confirm, false on cancel/Escape/backdrop. */
-export function useConfirm(): ConfirmFn {
-  const ask = useContext(ChoiceContext);
-  return useCallback(
-    async (opts: ConfirmOptions) => {
-      const picked = await ask({
-        title: opts.title,
-        message: opts.message,
-        cancelLabel: opts.cancelLabel,
-        choices: [{ id: "confirm", label: opts.confirmLabel ?? "", danger: opts.danger }],
-      });
-      return picked === "confirm";
-    },
-    [ask],
-  );
-}
-
-/** Three or more outcomes. Resolves to the chosen id, or **null** when the user cancels,
- * presses Escape or clicks the backdrop — dismissal is never one of the actions. */
-export function useChoice(): ChoiceFn {
-  return useContext(ChoiceContext);
-}
+// The dialog behind useConfirm()/useChoice() (see confirmContext.ts for the hooks and their contract).
 
 export function ConfirmProvider({ children }: { children: ReactNode }) {
   const { t } = useTranslation();