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[]) {