feat(ui): migrate the canonical action-button surface onto <Button>

Sweep the rounded-lg solid/ghost/outline action-button family onto the shared primitive:
ConfirmProvider, ErrorDialog, DownloadDialog, DownloadCenter, downloadModals, downloadAdmin,
ProfileEditor, PersonalViewsModal, SettingsPanel, WatchPage, DraftSaveBar. Deliberately left
on their own styles per the agreed scope: login/onboarding rounded-xl CTAs (Welcome/Onboarding/
Setup/KeyGate), glass Plex surfaces, segmented pills/toggles/switches/chips, dropdown triggers,
data-driven Banner/tone maps, text-xs dense buttons, and tiny inline p-1/p-1.5 icon controls —
none map 1:1 to a variant+size without a visual change, and the repo has no tailwind-merge.

frontend/src/components/downloadAdmin.tsx

@@ -8,6 +8,7 @@ import { useLiveQuery } from "../lib/useLiveQuery";
 import { api } from "../lib/api";
 import { formatBytes } from "../lib/format";
 import { inputCls } from "./ui/form";
+import { Button } from "./ui/Button";
 import { StatusPill, jobTitle } from "./downloadRow";
 
 // The admin "System" tab — storage overview, per-user footprint, quota editor, and the global job
@@ -110,19 +111,17 @@ function QuotaModal({
           {t("downloads.admin.unlimited")}
         </label>
         <div className="flex justify-between pt-1">
-          <button
-            onClick={() => reset.mutate()}
-            className="px-3 py-1.5 rounded-lg text-sm text-muted hover:text-fg hover:bg-surface transition"
-          >
+          <Button variant="ghost" size="sm" onClick={() => reset.mutate()}>
             {t("downloads.admin.reset")}
-          </button>
-          <button
+          </Button>
+          <Button
+            variant="primary"
+            size="sm"
             onClick={() => save.mutate()}
             disabled={save.isPending}
-            className="px-3 py-1.5 rounded-lg text-sm font-medium btn-primary disabled:opacity-50 transition"
           >
             {t("downloads.admin.save")}
-          </button>
+          </Button>
         </div>
       </div>
     </Modal>