refactor(ui): one rule for resettable, role key and display name; an honest reset label

The card menu's reset read 'Clear watch progress' but also un-hides and un-watches; it now says so.
The resettable rule, the demo-first role key and the display-name fallback each lived in several
components tied by comments; they are shared helpers now, and the menu reuses the card's own
strings instead of duplicate keys.

frontend/src/components/AdminUsers.tsx

@@ -23,6 +23,7 @@ import {
   type Me,
   type PendingAccessRow,
 } from "../lib/api";
+import { displayName } from "../lib/api/account";
 import { notify } from "../lib/notifications";
 import { formatDateTime, relativeTime } from "../lib/format";
 import { invalidateUserAccess } from "../lib/adminUsers";
@@ -234,7 +235,7 @@ function UsersRoles({ me }: { me: Me }) {
                   className="selectable cursor-pointer min-w-0 flex-1 text-left rounded-lg -m-1 p-1"
                 >
                   <div className="text-sm truncate">
-                    {u.display_name ?? u.email.split("@")[0]}
+                    {displayName(u)}
                     {self && <span className="text-muted"> · {t("users.roles.you")}</span>}
                   </div>
                   <div className="text-2xs text-muted truncate">{u.email}</div>