feat(pickers): a list that failed to load offers Retry

A picker whose options failed to load said so, but the only way back was closing its dialog.
Combobox takes `onRetry`: a Retry beside the message for the pointer, and Enter in the field for
the keyboard (the panel is a listbox, which may hold options only, so the alert names Enter for
assistive tech). Every caller with an error state passes its query's refetch: both user pickers,
the channel picker and the two Plex pickers.

frontend/src/components/PlexSidebar.tsx

@@ -373,6 +373,7 @@ function PlaylistPicker() {
       placeholder={t("plex.playlist.pickerPlaceholder")}
       loading={playlistsQ.isLoading}
       error={playlistsQ.isError && !playlistsQ.data ? t("common.loadError") : undefined}
+      onRetry={() => void playlistsQ.refetch()}
       emptyText={t("plex.playlist.none")}
       renderOption={(o) =>
         o.value === CREATE_PLAYLIST ? (
@@ -457,6 +458,7 @@ function CollectionPicker({
         onOpenChange={setOpen}
         loading={q.isFetching}
         error={q.isError ? t("common.loadError") : undefined}
+        onRetry={() => void q.refetch()}
         placeholder={t("plex.filter.collectionSearch")}
         emptyText={t("plex.filter.collectionEmpty")}
         onChange={(next) => {