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/DownloadDialog.tsx

@@ -3,6 +3,7 @@ import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import Modal from "./Modal";
+import { Button } from "./ui/Button";
 import { api } from "../lib/api";
 import { invalidateDownloads } from "../lib/downloads";
 
@@ -89,19 +90,12 @@ export default function DownloadDialog({
       />
 
       <div className="flex justify-end gap-2">
-        <button
-          onClick={onClose}
-          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={onClose}>
           {t("downloads.actions.cancel")}
-        </button>
-        <button
-          onClick={submit}
-          disabled={busy || !chosen}
-          className="px-3 py-1.5 rounded-lg text-sm font-medium btn-primary disabled:opacity-50 transition"
-        >
+        </Button>
+        <Button variant="primary" size="sm" onClick={submit} disabled={busy || !chosen}>
           {t("downloads.dialog.submit")}
-        </button>
+        </Button>
       </div>
 
       {manage && (