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.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,
+});