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/DownloadCenter.tsx

@@ -31,6 +31,7 @@ import { useMe } from "../lib/useMe";
 import { api, type DownloadJob } from "../lib/api";
 import { invalidateDownloads } from "../lib/downloads";
 import { inputCls } from "./ui/form";
+import { Button } from "./ui/Button";
 import { DownloadRow, IconBtn, UsageBar } from "./downloadRow";
 import { MetaEditModal, ErrorModal } from "./downloadModals";
 import { AdminSystem } from "./downloadAdmin";
@@ -169,20 +170,24 @@ export default function DownloadCenter() {
                 placeholder={t("downloads.page.addUrl")}
                 className={inputCls}
               />
-              <button
+              <Button
+                variant="ghost"
+                size="sm"
+                iconOnly
+                icon={<ClipboardPaste className="w-4 h-4" />}
+                label={t("downloads.page.paste")}
                 onClick={() => pasteFromClipboard(true)}
-                title={t("downloads.page.paste")}
-                aria-label={t("downloads.page.paste")}
-                className="inline-flex items-center justify-center px-2.5 py-1.5 rounded-lg text-muted hover:text-fg hover:bg-surface transition shrink-0"
-              >
-                <ClipboardPaste className="w-4 h-4" />
-              </button>
-              <button
+                className="shrink-0"
+              />
+              <Button
+                variant="primary"
+                size="sm"
+                icon={<Plus className="w-4 h-4" />}
                 onClick={() => addUrl.trim() && setAddSource(addUrl.trim())}
-                className="inline-flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm font-medium btn-primary transition shrink-0"
+                className="shrink-0"
               >
-                <Plus className="w-4 h-4" /> {t("downloads.page.add")}
-              </button>
+                {t("downloads.page.add")}
+              </Button>
             </div>
             <div className="space-y-2">
               {queue.map((job) => (