Bring the same tag control to the channel page, next to Subscribe:
- backend: GET /api/channels/{id} now returns this user's `tag_ids` for the channel
(added to _channel_detail_dict + the ChannelDetail interface), matching the manager
list shape;
- ChannelPage renders the shared ChannelTagsControl, wired to attach/detach/create with
an optimistic update on the channel-detail cache (reverting on failure) and a manager
list invalidation so a tag added here shows there too.
Reuses the R8 popover platform + ChannelTagsControl — no new tag UI.
peter
56f95fa626