feat(e10): finish the mappable Button migration, including the ui/ layer itself

Third sweep, 20 more sites: the rest of AdminUsers (the access-request
and invite clusters), Scheduler, SettingsPanel, SetupWizard, About, Feed,
KeyGate, OnboardingWizard -- and, importantly, four platform components
that were still hand-rolling their own buttons: Modal's close, Banner's
dismiss, SidePanel's two toggles and QueryState's retry. A platform layer
that bypasses its own primitive is the same defect one level up.

Hand-rolled buttons with their own geometry: 110 -> 39; Button call sites
37 -> 106. What is left is deliberate, and it is the list Button's own
docstring excludes plus two families this pass refused to force: video
overlay controls painted on white/N, drag handles, chip removers,
accent-solid pills, the bg-card family on the signed-out screen, and
sites whose paint is conditional (`suspended ? warn : neutral`) or
otherwise has no variant yet. Forcing those through a className override
would be unsafe without tailwind-merge, which is exactly the hazard this
sprint documented.

3x tsc, eslint, knip, prettier, 689 vitest green; UAT on Settings and the
About dialog (its close button and full-width CTA are both migrated),
clean console.

frontend/src/components/About.tsx

@@ -4,6 +4,7 @@ import { useTranslation } from "react-i18next";
 import { Info, Sparkles } from "lucide-react";
 import { api } from "../lib/api";
 import { FRONTEND_VERSION, FRONTEND_SHA, FRONTEND_BUILD_DATE } from "../lib/version";
+import { Button } from "./ui/Button";
 import Modal from "./ui/Modal";
 
 function fmtDate(d?: string | null): string {
@@ -70,12 +71,9 @@ export default function About({
         ))}
       </div>
 
-      <button
-        onClick={onOpenReleaseNotes}
-        className="mt-4 w-full inline-flex items-center justify-center gap-2 px-4 py-2 rounded-xl font-semibold btn-primary transition"
-      >
+      <Button variant="primary" size="lg" className="mt-4 w-full" onClick={onOpenReleaseNotes}>
         <Sparkles className="w-4 h-4" /> {t("about.whatsNew")}
-      </button>
+      </Button>
     </Modal>
   );
 }