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/KeyGate.tsx

@@ -154,14 +154,10 @@ export default function KeyGate({ meId, compact = false }: { meId: number; compa
         />
       )}
       {err && <div className="text-sm text-danger">{err}</div>}
-      <button
-        onClick={submit}
-        disabled={busy || !pass}
-        className="inline-flex items-center gap-2 px-4 py-2 rounded-xl font-semibold btn-primary transition disabled:opacity-40"
-      >
+      <Button variant="primary" size="md" onClick={submit} disabled={busy || !pass}>
         <Lock className="w-4 h-4" />
         {configured ? t("messages.unlock") : t("messages.setUp")}
-      </button>
+      </Button>
       {configured && (
         <button onClick={toReset} className="block text-xs text-muted hover:text-fg">
           {t("messages.forgotPass")}