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/SettingsPanel.tsx

@@ -495,13 +495,15 @@ function SignInMethods({ me }: { me: Me }) {
               {t("settings.account.sessions.hint")}
             </p>
           </div>
-          <button
+          <Button
+            variant="glass"
+            size="sm"
+            className="shrink-0"
             onClick={logoutOthers}
             disabled={sessBusy}
-            className="shrink-0 glass-card glass-hover px-3 py-1.5 rounded-xl text-sm transition disabled:opacity-40"
           >
             {t("settings.account.sessions.action")}
-          </button>
+          </Button>
         </div>
       </div>
     </Section>
@@ -830,13 +832,10 @@ function Account({ me, onOpenWizard }: { me: Me; onOpenWizard: () => void }) {
           <p className="text-xs text-muted leading-relaxed mb-2">
             {t("settings.account.deleteHint")}
           </p>
-          <button
-            onClick={onDeleteAccount}
-            className="inline-flex items-center gap-1.5 px-3 py-2 rounded-xl text-sm border border-danger/40 text-danger hover:bg-danger/10 transition"
-          >
+          <Button variant="danger-soft" size="md" onClick={onDeleteAccount}>
             <Trash2 className="w-4 h-4" />
             {t("settings.account.deleteAccount")}
-          </button>
+          </Button>
         </Section>
       )}
     </>