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/Scheduler.tsx

@@ -19,6 +19,7 @@ import {
 } from "lucide-react";
 import { api, type SchedulerJob, type SchedulerStatus } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
+import { Button } from "./ui/Button";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 import { useConfirm } from "./ConfirmProvider";
 import { relativeTime } from "../lib/format";
@@ -283,13 +284,9 @@ function MaintenanceSettings({
           aria-label={t("scheduler.maintenance.batchLabel")}
           className="w-28 bg-surface border border-border rounded-sm px-2 py-1 text-sm tabular-nums focus:outline-hidden focus:ring-2 focus:ring-accent/40"
         />
-        <button
-          onClick={save}
-          disabled={!dirty || saving}
-          className="glass-card glass-hover px-3 py-1.5 rounded-lg text-sm disabled:opacity-40 transition"
-        >
+        <Button variant="glass" size="sm" onClick={save} disabled={!dirty || saving}>
           {t("common.save")}
-        </button>
+        </Button>
       </div>
       <div className="text-2xs text-muted mt-1.5">
         {t("scheduler.maintenance.batchNote", {