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/components/channelColumns.dom.test.tsx

@@ -5,8 +5,8 @@
 // "fully" | "needsFull" | "" vocab + predicate — asserted here so a regression can't silently change
 // which channels the top chips show.
 import { describe, it, expect } from "vitest";
-import { applySyncFilter } from "./channelColumns";
-import { type ManagedChannel } from "../lib/api";
+import { applySyncFilter, tagsColumnTest } from "./channelColumns";
+import { NO_TAG_SENTINEL, type ManagedChannel } from "../lib/api";
 
 const ch = (over: Partial<ManagedChannel>): ManagedChannel => ({
   id: "c",
@@ -51,3 +51,23 @@ describe("applySyncFilter", () => {
     expect(applySyncFilter(rows, "needsFull").map((r) => r.id)).toEqual(["recentOnly", "none"]);
   });
 });
+
+describe("tagsColumnTest (item B — 'No tag')", () => {
+  const tagged = ch({ id: "tagged", tag_ids: [1, 2] });
+  const untagged = ch({ id: "untagged", tag_ids: [] });
+
+  it("the No-tag sentinel keeps only channels with no user tag", () => {
+    expect(tagsColumnTest(tagged, [NO_TAG_SENTINEL])).toBe(false);
+    expect(tagsColumnTest(untagged, [NO_TAG_SENTINEL])).toBe(true);
+  });
+
+  it("real tag values match ANY selected id (sentinel absent)", () => {
+    expect(tagsColumnTest(tagged, ["2"])).toBe(true);
+    expect(tagsColumnTest(tagged, ["3"])).toBe(false);
+    expect(tagsColumnTest(untagged, ["1"])).toBe(false);
+  });
+
+  it("the sentinel wins if it somehow co-exists with real ids", () => {
+    expect(tagsColumnTest(tagged, [NO_TAG_SENTINEL, "1"])).toBe(false);
+  });
+});