refactor(design): one drawn check for every checkable row that cannot be an input (E28 S3e3b)

ui/MenuCheck: a box filled with the accent for a set (a minus for a group
only partly in), a bare tick for one choice, in the row's leading slot.
Replaces seven hand-drawn marks: three box sizes, a tinted box, a cross
for "on" in the table filter, a trailing tick in the language menu.

frontend/src/components/LanguageSwitcher.tsx

@@ -1,5 +1,6 @@
 import { useState } from "react";
-import { Check, Globe } from "lucide-react";
+import { Globe } from "lucide-react";
+import { MenuCheck } from "./ui/MenuCheck";
 import { LANGUAGES, type LangCode } from "../i18n";
 import { PopoverItem, PopoverSurface } from "./ui/AnchoredPopover";
 import { useAnchoredPopover } from "./ui/anchoredPopoverHooks";
@@ -83,10 +84,10 @@ export default function LanguageSwitcher({
               setOpen(false);
             }}
             label={l.label}
-            className="w-full flex items-center justify-between gap-2 text-sm px-2 py-2 rounded-lg text-muted outline-hidden transition hover:text-fg"
+            className="w-full flex items-center gap-2 text-sm px-2 py-2 rounded-lg text-muted outline-hidden transition hover:text-fg"
           >
+            <MenuCheck kind="one" on={l.code === value} />
             <span>{l.label}</span>
-            {l.code === value && <Check className="w-4 h-4 text-accent" />}
           </PopoverItem>
         ))}
       </PopoverSurface>