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/pagerContent.ts

@@ -0,0 +1,14 @@
+// The pager's page arithmetic, split out of Pager so that file exports only components (Fast Refresh)
+// and table chrome can gate on it without importing the control.
+
+/** How many pages `total` rows make at `size` (`size <= 0` = "All" = one page). */
+export function pageCount(total: number, size: number): number {
+  return size <= 0 ? 1 : Math.max(1, Math.ceil(total / size));
+}
+
+/** Whether a pager would render anything — the ONE owner of that rule. Callers that wrap the pager
+ * in their own chrome gate on this instead of re-deriving it: a drifted second copy is exactly what
+ * left empty control bands behind on tables with nothing to page. */
+export function hasPagerContent(total: number, size: number): boolean {
+  return pageCount(total, size) > 1 || total > 0;
+}