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/ErrorDialog.tsx

@@ -2,6 +2,7 @@ import { useSyncExternalStore } from "react";
 import { useTranslation } from "react-i18next";
 import { dismissError, getError, subscribeError } from "../lib/errorDialog";
 import Modal from "./Modal";
+import { Button } from "./ui/Button";
 
 // Renders the current app error (if any) as a modal the user must acknowledge.
 export default function ErrorDialog() {
@@ -12,12 +13,7 @@ export default function ErrorDialog() {
     <Modal title={err.title} onClose={dismissError} maxWidth="max-w-md">
       <p className="text-sm leading-relaxed text-fg/90">{err.message}</p>
       <div className="flex justify-end mt-5">
-        <button
-          onClick={dismissError}
-          className="px-4 py-2 rounded-lg btn-primary text-sm font-medium transition"
-        >
-          {t("errors.ok")}
-        </button>
+        <Button onClick={dismissError}>{t("errors.ok")}</Button>
       </div>
     </Modal>
   );