refactor(ui): FilterSelect and Combobox draw their checks with MenuCheck

They still painted their own 14px box beside the menus' 16px MenuCheck. A contract test holds the
filled-box paint to MenuCheck alone.

frontend/src/components/ui/menuCheck.contract.test.ts

@@ -0,0 +1,19 @@
+import { readdirSync, readFileSync } from "node:fs";
+import { describe, expect, it } from "vitest";
+
+// E28 review L2: `ui/MenuCheck` is the ONE drawn check box of a checkable row (S3e3b replaced seven
+// hand-drawn copies), yet FilterSelect and Combobox still painted their own — a 14px box beside the
+// menus' 16px one. The filled-box paint is the tell: it may live in MenuCheck only.
+describe("the drawn check box", () => {
+  it("is painted by MenuCheck alone", () => {
+    const root = new URL("../", import.meta.url);
+    const offenders = readdirSync(root, { recursive: true, encoding: "utf8" })
+      .filter((f) => f.endsWith(".tsx") && !f.includes(".test.") && !f.endsWith("MenuCheck.tsx"))
+      .filter((f) =>
+        /border-accent bg-accent text-accent-fg/.test(
+          readFileSync(new URL(f.replace(/\\/g, "/"), root), "utf8"),
+        ),
+      );
+    expect(offenders).toEqual([]);
+  });
+});