fix(ui): the filter rail shows no tag counts until the facets land

It printed the tag's whole-library count as the current filter's while the
facets loaded, and for good when they failed: live, "Music 238" beside a
search that matched 3 of its channels. Ordering still falls back to it.

frontend/src/components/facetOption.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from "vitest";
+
+import type { Tag } from "../lib/api";
+import { facetOption } from "./facetOption";
+
+const music = { id: 7, name: "Music", category: "topic", channel_count: 238 } as Tag;
+
+describe("a tag as a filter row", () => {
+  it("counts the current filter's channels", () => {
+    expect(facetOption(music, { "7": 3 }).count).toBe(3);
+    expect(facetOption(music, {}).count).toBe(0);
+  });
+
+  // E31 review 2, live: with the summary failed, the rail read "Music 238" beside a search that matched
+  // 3 Music channels — the whole library's count printed as the filter's.
+  it("shows no count until the facets land, rather than the whole library's", () => {
+    expect(facetOption(music, undefined).count).toBeUndefined();
+  });
+});