fix(ui): a picker whose list failed says so, and takes no pick while one is being written

- Combobox gains `error` (said in the list, not "No matches") and a soft `disabled` (keeps the
  focus, announced, opens and changes nothing).
- UserPicker passes both: the share dialog's recipient picker is off while a share is pending, and
  the share and quota pickers say when the user list failed to load.
- ChannelPicker and the Plex playlist/collection pickers say a failed load too.
- The channel menu's Tags row says loading / failed instead of "no tags yet".

frontend/src/components/PlexSidebar.tsx

@@ -371,6 +371,8 @@ function PlaylistPicker() {
       query={query}
       onQueryChange={setQuery}
       placeholder={t("plex.playlist.pickerPlaceholder")}
+      loading={playlistsQ.isLoading}
+      error={playlistsQ.isError && !playlistsQ.data ? t("common.loadError") : undefined}
       emptyText={t("plex.playlist.none")}
       renderOption={(o) =>
         o.value === CREATE_PLAYLIST ? (
@@ -454,6 +456,7 @@ function CollectionPicker({
         onQueryChange={setSearch}
         onOpenChange={setOpen}
         loading={q.isFetching}
+        error={q.isError ? t("common.loadError") : undefined}
         placeholder={t("plex.filter.collectionSearch")}
         emptyText={t("plex.filter.collectionEmpty")}
         onChange={(next) => {