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/Feed.tsx

@@ -38,6 +38,7 @@ import { FEED_VIEWS, type FeedView } from "../lib/feedView";
 import { clearedFilters, useActiveFilters } from "../lib/useActiveFilters";
 import ActiveFilterChips from "./ActiveFilterChips";
 import SelectedViewChip from "./SelectedViewChip";
+import { Button } from "./ui/Button";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 import { PageToolbar } from "./ui/PageShell";
 import { useWizard } from "./WizardProvider";
@@ -530,20 +531,18 @@ export default function Feed({
             {isDemo ? t("feed.demoEmptyBody") : t("feed.emptyBody")}
           </p>
           {isDemo ? (
-            <button
+            <Button
+              variant="primary"
+              size="lg"
               onClick={() => setFilters({ ...filters, scope: "all" })}
-              className="inline-flex items-center justify-center gap-2 px-5 py-2.5 rounded-xl font-semibold btn-primary transition"
             >
               {t("feed.browseLibrary")}
-            </button>
+            </Button>
           ) : (
             <>
-              <button
-                onClick={onOpenWizard}
-                className="inline-flex items-center justify-center gap-2 px-5 py-2.5 rounded-xl font-semibold btn-primary transition"
-              >
+              <Button variant="primary" size="lg" onClick={onOpenWizard}>
                 {t("feed.setUp")}
-              </button>
+              </Button>
               <button
                 onClick={() => setFilters({ ...filters, scope: "all" })}
                 className="block mx-auto mt-3 text-sm text-muted hover:text-accent transition"