feat(feed,channels): "No tag" filter for untagged channels (E8·S1)

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.

frontend/src/lib/api.ts

@@ -32,7 +32,13 @@ export type * from "./api/setup";
 import { feedApi } from "./api/feed";
 export type * from "./api/feed";
 export type * from "./api/tagModes";
-export { DEFAULT_TAG_MODES, TAG_CATEGORIES, coerceTagModes } from "./api/tagModes";
+export {
+  DEFAULT_TAG_MODES,
+  TAG_CATEGORIES,
+  NO_TAG_SENTINEL,
+  coerceTagModes,
+  coerceUntagged,
+} from "./api/tagModes";
 import { channelsApi } from "./api/channels";
 export type * from "./api/channels";
 import { syncApi } from "./api/sync";