fix(web): place aria-selected on the role=tab element (#32) + assert it

Per WAI-ARIA, aria-selected must be on the same element carrying role="tab".
The previous impl placed it on an inner <span> via the render-prop, making it
invisible to assistive technology. Move both role="tab" and aria-selected to
the NavLink, compute currentKind once from useParams, drop the render-prop.
Add a Vitest assertion that the selected tab has aria-selected="true" and an
unselected tab has aria-selected="false".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/authorities/authorities-page.tsx

@@ -18,8 +18,9 @@ export function AuthoritiesPage() {
   const lang = i18n.language.startsWith("sv") ? "sv" : "en";
 
   const isValidKind = (KINDS as readonly string[]).includes(kind ?? "");
+  const currentKind = isValidKind ? (kind as string) : "person";
 
-  const { data: authorities, isLoading, isError } = useAuthorities(isValidKind ? (kind as string) : "person");
+  const { data: authorities, isLoading, isError } = useAuthorities(currentKind);
   const create = useCreateAuthority();
 
   const [labels, setLabels] = useState<LabelInput[]>([]);
@@ -50,11 +51,12 @@ export function AuthoritiesPage() {
             key={k}
             to={`/authorities/${k}`}
             role="tab"
+            aria-selected={k === currentKind}
             className={({ isActive }) =>
               `rounded px-3 py-1 text-sm ${isActive ? "bg-neutral-800 text-white" : "border"}`
             }
           >
-            {({ isActive }) => <span aria-selected={isActive}>{t(`authorities.${k}`)}</span>}
+            {t(`authorities.${k}`)}
           </NavLink>
         ))}
       </div>
@@ -78,7 +80,7 @@ export function AuthoritiesPage() {
 
       <form onSubmit={onCreate} className="space-y-2 border-t pt-3">
         <div className="text-sm font-medium">
-          {t("authorities.new")} · {t(`authorities.${kind}`)}
+          {t("authorities.new")} · {t(`authorities.${currentKind}`)}
         </div>
 
         <LabelEditor value={labels} onChange={setLabels} />