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/Combobox.tsx

@@ -1,7 +1,8 @@
 import clsx from "clsx";
 import { useCallback, useEffect, useMemo, useRef, useState, type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
-import { Check, ChevronDown } from "lucide-react";
+import { ChevronDown } from "lucide-react";
+import { MenuCheck } from "./MenuCheck";
 import { PopoverSurface, PopoverItem } from "./AnchoredPopover";
 import { useAnchoredPopover } from "./anchoredPopoverHooks";
 import { useScrollFade } from "../../lib/useScrollFade";
@@ -285,13 +286,7 @@ export default function Combobox({
                       renderOption(o, on)
                     ) : (
                       <>
-                        <span
-                          className={`flex h-3.5 w-3.5 shrink-0 items-center justify-center rounded border ${
-                            on ? "border-accent bg-accent text-accent-fg" : "border-border"
-                          }`}
-                        >
-                          {on && <Check className="h-2.5 w-2.5" />}
-                        </span>
+                        <MenuCheck kind={multiple ? "many" : "one"} on={on} />
                         <span className="flex-1 truncate">{o.label ?? o.value}</span>
                         {o.count != null && (
                           <span className="shrink-0 text-2xs tabular-nums text-muted">