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/SetupWizard.tsx

@@ -4,6 +4,7 @@ import { Check, ChevronLeft, ChevronRight, Loader2, Send } from "lucide-react";
 import { api, HttpError } from "../lib/api";
 import { setLanguage, type LangCode } from "../i18n";
 import LanguageSwitcher from "./LanguageSwitcher";
+import { Button } from "./ui/Button";
 import { inputCls } from "./ui/form";
 
 // First-run install wizard. Shown by App.tsx while the instance is unconfigured. The one-time
@@ -239,15 +240,17 @@ export default function SetupWizard() {
                     placeholder={t("setup.smtp.testTo")}
                     className={inputCls}
                   />
-                  <button
+                  <Button
+                    variant="glass"
+                    size="md"
+                    className="shrink-0"
                     type="button"
                     onClick={sendTest}
                     disabled={testState === "sending" || !testTo.trim()}
-                    className="shrink-0 glass-card glass-hover inline-flex items-center gap-1.5 px-3 py-2 rounded-xl text-sm disabled:opacity-50 transition"
                   >
                     <Send className={`w-4 h-4 ${testState === "sending" ? "animate-pulse" : ""}`} />
                     {t("setup.smtp.test")}
-                  </button>
+                  </Button>
                 </div>
               )}
               {testState === "ok" && (