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/Welcome.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useRef, useState } from "react";
-import { useHistorySubview, type SubviewUrl } from "../lib/history";
+import { useHistorySubview } from "../lib/history";
 import { useTranslation } from "react-i18next";
 import {
   ArrowRight,
@@ -34,24 +34,11 @@ import { anyBlockingLayerAbove, isTopmost, pushLayer, removeLayer } from "../lib
 import { inputCls } from "./ui/form";
 import { entryHash, entrySearch } from "../lib/entryUrl";
 import type { VerifyEntry } from "../lib/useVerifyEntry";
+import { MODE_URL, type Mode } from "./welcomeMode";
 
 const EMAIL_RE = /^[^@\s]+@[^@\s]+\.[^@\s]+$/;
 const PASSWORD_MIN = 10;
 
-export type Mode = "signin" | "register" | "forgot" | "reset" | "resend";
-
-// The card's sub-views are real history entries, so browser/mouse Back leaves one the way it leaves
-// any other page — they used to be plain state, and Back silently changed only the address bar.
-// "signin" is the root (no param) and "reset" is deliberately NOT mirrored: that view is only
-// reachable with the one-time token from the #reset fragment, so a bookmarked ?m=reset would open a
-// form that cannot submit.
-export const MODE_URL: SubviewUrl<Mode> = {
-  key: "m",
-  encode: (m: Mode) => (m === "signin" || m === "reset" ? null : m),
-  decode: (raw: string) =>
-    raw === "register" || raw === "forgot" || raw === "resend" ? (raw as Mode) : null,
-};
-
 // Per-mode i18n keys for the card title and the submit button. A flat map (vs nested ternaries)
 // keeps adding a new mode a one-line change and makes a missing key a compile error.
 const TITLE_KEY: Record<Mode, string> = {