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/PlexViewBridge.tsx

@@ -1,5 +1,9 @@
-import { createContext, useContext, useState, type ReactNode } from "react";
-import { type Sub } from "./plexSubUrl";
+import { useState, type ReactNode } from "react";
+import {
+  PlexViewPublishContext,
+  PlexViewReadContext,
+  type PlexViewControls,
+} from "./plexViewContext";
 
 // A one-way bridge so the always-mounted Header can host the Plex Discover ⟷ Browse toggle even though
 // the sub-view state lives in (and is owned by) the plex-only PlexBrowse. PlexBrowse PUBLISHES its
@@ -11,22 +15,14 @@ import { type Sub } from "./plexSubUrl";
 // each plex entry starts at the Discover root). Lifting that hook to an always-mounted provider would
 // lose the remount and let the view go stale across navigation. Keeping it in PlexBrowse preserves the
 // reseed; the bridge only mirrors the small slice the header needs.
-export type PlexViewControls = {
-  kind: Sub["kind"]; // the current top-level plex view — the header controls derive their state from it
-  open: (v: Sub) => void;
-};
-
-const ReadCtx = createContext<PlexViewControls | null>(null);
-const PublishCtx = createContext<(c: PlexViewControls | null) => void>(() => {});
+//
+// The contexts + hooks live in plexViewContext.ts.
 
 export function PlexViewBridge({ children }: { children: ReactNode }) {
   const [controls, setControls] = useState<PlexViewControls | null>(null);
   return (
-    <PublishCtx.Provider value={setControls}>
-      <ReadCtx.Provider value={controls}>{children}</ReadCtx.Provider>
-    </PublishCtx.Provider>
+    <PlexViewPublishContext.Provider value={setControls}>
+      <PlexViewReadContext.Provider value={controls}>{children}</PlexViewReadContext.Provider>
+    </PlexViewPublishContext.Provider>
   );
 }
-
-export const usePlexViewControls = () => useContext(ReadCtx);
-export const usePublishPlexView = () => useContext(PublishCtx);