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/ui/Banner.tsx

@@ -1,6 +1,7 @@
 import type { ReactNode } from "react";
 import type { LucideIcon } from "lucide-react";
 import { X } from "lucide-react";
+import { Button } from "./Button";
 
 type BannerTone = "accent" | "warning";
 
@@ -56,13 +57,14 @@ export default function Banner({
         </button>
       )}
       {onDismiss && (
-        <button
+        <Button
+          variant="ghost"
+          size="icon-sm"
+          className="ml-auto"
           onClick={onDismiss}
           title={dismissTitle}
-          className="ml-auto p-1 rounded-md text-muted hover:text-fg hover:bg-card transition"
-        >
-          <X className="w-4 h-4" />
-        </button>
+          icon={<X className="w-4 h-4" />}
+        />
       )}
     </div>
   );