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/Sidebar.tsx

@@ -14,9 +14,10 @@ import FilterRail from "./FilterRail";
 import { type PanelItem } from "./ui/PanelGroups";
 import { type MediaSourceKind } from "../lib/mediaSource";
 import ChannelPicker from "./ChannelPicker";
+import { facetOption } from "./facetOption";
 import DateFilterSelect from "./DateFilterSelect";
 import YearFilter from "./YearFilter";
-import Combobox, { type ComboboxOption } from "./ui/Combobox";
+import Combobox from "./ui/Combobox";
 import MatchToggle from "./ui/MatchToggle";
 import { useFeedFilters, useFeedFiltersActions } from "./feedFiltersContext";
 import { useChannelsActions } from "./channelsContext";
@@ -72,6 +73,8 @@ export default function Sidebar({
   });
   const facetsReady = !!summaryQuery.data;
   const facetCounts = summaryQuery.data?.facets ?? {};
+  // Ordering falls back to the tag's whole-library count until the facets land; the number shown does
+  // not (facetOption).
   const chipCount = (tag: Tag): number =>
     facetsReady ? (facetCounts[String(tag.id)] ?? 0) : tag.channel_count;
   const visibleChips = (list: Tag[]): Tag[] => {
@@ -93,11 +96,7 @@ export default function Sidebar({
 
   // The language/topic/tags sections all write into the one `filters.tags` list, so each combobox owns
   // only its own slice: its selection replaces the ids that belong to its list, leaving the others.
-  const tagOption = (tg: Tag): ComboboxOption => ({
-    value: String(tg.id),
-    label: tg.name,
-    count: chipCount(tg),
-  });
+  const tagOption = (tg: Tag) => facetOption(tg, summaryQuery.data?.facets);
   const selectedIn = (list: Tag[]): string[] =>
     filters.tags.filter((id) => list.some((tg) => tg.id === id)).map(String);
   function setTagSelection(list: Tag[], next: string[]) {

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();
+  });
+});

frontend/src/components/facetOption.ts

@@ -0,0 +1,14 @@
+import type { Tag } from "../lib/api";
+import type { ComboboxOption } from "./ui/Combobox";
+
+// A tag as a filter row: its count is the CURRENT filter's (the summary's facets), or none at all while
+// the facets load or after they failed. The rail printed the tag's whole-library count in that gap, as
+// if it were the filter's: "Music 238" beside a search that matched 3 of its channels (E31 review 2).
+export const facetOption = (
+  tag: Tag,
+  facets: Record<string, number> | undefined,
+): ComboboxOption => ({
+  value: String(tag.id),
+  label: tag.name,
+  count: facets ? (facets[String(tag.id)] ?? 0) : undefined,
+});