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/ChannelPicker.tsx

@@ -41,6 +41,7 @@ export default function ChannelPicker({
       onChange={(channelIds) => setFilters({ ...filters, channelIds })}
       loading={channelsQuery.isLoading}
       error={channelsQuery.isError && !channelsQuery.data ? t("common.loadError") : undefined}
+      onRetry={() => void channelsQuery.refetch()}
       emptyText={t("sidebar.noChannelMatch")}
       selectedLabels={selectedLabels}
     />