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/Channels.tsx

@@ -329,6 +329,7 @@ export default function Channels() {
     onView,
     onOpenWizard,
     tags: userTags,
+    tagsState: tagsQuery.isPending ? "loading" : tagsQuery.isError ? "error" : "ready",
     onToggleTag: (c, tagId) => toggleTag(c.id, tagId, c.tag_ids.includes(tagId)),
     onPickPriority: pickPriority,
   });