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/ShareDialog.tsx

@@ -46,6 +46,8 @@ function UserShare({ job }: { job: DownloadJob }) {
       <UserPicker
         users={list}
         loading={recipients.isLoading}
+        error={recipients.isError}
+        disabled={share.isPending}
         onPick={(u) => {
           if (!share.isPending) share.mutate(u.email);
         }}