Filter to channels you haven't personally tagged in the channel manager
(no user-owned ChannelTag; auto language/topic tags don't count):
- Feed: a "No tag" sentinel in the "Your tags" sidebar combobox (exclusive
with real tag picks) + a removable "Untagged" chip; new `untagged` bool
feed param / `notag` URL key, persisted + coerced on every filter path.
- Channel manager: a virtual "No tag" chip in the tag-chip row filters the
table to untagged channels (client-side, shared NO_TAG_SENTINEL).
Backend `untagged` is a plain channel-level narrowing, so feed_summary keeps
it applied in the base pass — the count/facet lockstep holds with no mirror.
Note: item E (per-tag counts) and item D (Any/All toggle) were already
shipped; S1 delivers only item B.
peter
4ee0dab68f