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/OnboardingWizard.tsx

@@ -5,6 +5,7 @@ import { qk } from "../lib/queryKeys";
 import { AlertTriangle, Check, Loader2, RefreshCw, ShieldCheck, X, Youtube } from "lucide-react";
 import { api, type Me } from "../lib/api";
 import { beginGrant, endOnboarding } from "../lib/onboarding";
+import { Button } from "./ui/Button";
 import Overlay from "./ui/Overlay";
 
 // A first-login wizard that walks the user through granting YouTube access one scope at a
@@ -159,12 +160,9 @@ export default function OnboardingWizard({ me, onClose }: { me: Me; onClose: ()
                 >
                   <Youtube className="w-4 h-4" /> {t("onboarding.read.connect")}
                 </button>
-                <button
-                  onClick={close}
-                  className="px-5 py-2 rounded-xl text-sm text-muted hover:text-fg transition"
-                >
+                <Button variant="ghost" size="md" onClick={close}>
                   {t("onboarding.read.skip")}
-                </button>
+                </Button>
               </div>
             </>
           )}
@@ -225,15 +223,16 @@ export default function OnboardingWizard({ me, onClose }: { me: Me; onClose: ()
                 >
                   <ShieldCheck className="w-4 h-4" /> {t("onboarding.write.enableUnsubscribe")}
                 </button>
-                <button
+                <Button
+                  variant="ghost"
+                  size="md"
                   onClick={() => {
                     endOnboarding(false);
                     onClose();
                   }}
-                  className="px-5 py-2 rounded-xl text-sm text-muted hover:text-fg transition"
                 >
                   {t("onboarding.write.keepReadOnly")}
-                </button>
+                </Button>
               </div>
             </>
           )}