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/ChannelPicker.tsx

@@ -40,6 +40,7 @@ export default function ChannelPicker({
       selected={filters.channelIds}
       onChange={(channelIds) => setFilters({ ...filters, channelIds })}
       loading={channelsQuery.isLoading}
+      error={channelsQuery.isError && !channelsQuery.data ? t("common.loadError") : undefined}
       emptyText={t("sidebar.noChannelMatch")}
       selectedLabels={selectedLabels}
     />