fix(groups): hover affordance on the remove-member button

Reported: the new "移出" (remove member) button gave no visual
feedback on hover, so it didn't read as clickable. Tailwind's own
preflight reset sets buttons to cursor:default (not the browser's
usual pointer) unless a cursor utility says otherwise -- adds
cursor-pointer + hover:underline, verified live via
getComputedStyle/:hover that both apply while hovering and correctly
reset on the disabled (in-flight) state.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/features/groups/GroupMembersModal.tsx

@@ -214,7 +214,7 @@ export function GroupMembersModal({ topicId, canInvite, myUserId, myRole, initia
                         removeMutation.mutate(m.user_id);
                       }
                     }}
-                    className="flex-none text-xs text-danger-text disabled:opacity-60"
+                    className="flex-none cursor-pointer text-xs text-danger-text hover:underline disabled:pointer-events-none disabled:cursor-default disabled:opacity-60 disabled:hover:no-underline"
                   >
                     {removeMutation.isPending && removeMutation.variables === m.user_id ? '移出中…' : '移出'}
                   </button>