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/downloadRow.tsx

@@ -17,6 +17,7 @@ import {
 } from "../lib/format";
 import { navItemAttrs } from "../lib/listNav";
 import { failCopyKeys } from "../lib/downloadFailure";
+import { jobTitle } from "./downloadTitle";
 
 // Download-row presentation + the small shared primitives (status pill, icon button, usage bar),
 // split out of DownloadCenter (all purely prop-driven, no shared state). Rendered by the queue,
@@ -104,10 +105,6 @@ function Thumb({ job }: { job: DownloadJob }) {
   );
 }
 
-export function jobTitle(job: DownloadJob): string {
-  return job.display_name || job.asset?.title || job.source_ref;
-}
-
 // The channel to show under the title: a user override wins over the auto-extracted asset value.
 // `url` is null for sources without a channel link (e.g. reddit) — then it renders as plain text.
 function jobChannel(job: DownloadJob): { name: string; url: string | null } {