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/SettingsPanel.tsx

@@ -11,6 +11,7 @@ import Avatar from "./Avatar";
 import { notify, type NotifSettings } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import { inputCls, Section, SettingRow, Switch } from "./ui/form";
+import { Button } from "./ui/Button";
 import { type SaveState } from "./ui/DraftSaveBar";
 import { useConfirm } from "./ConfirmProvider";
 import { useWizard } from "./WizardProvider";
@@ -472,13 +473,9 @@ function SignInMethods({ me }: { me: Me }) {
               className={inputCls}
             />
             {err && <p className="text-xs text-danger">{err}</p>}
-            <button
-              type="submit"
-              disabled={busy || !next}
-              className="px-3 py-1.5 rounded-lg text-sm btn-primary font-medium disabled:opacity-40 transition"
-            >
+            <Button type="submit" variant="primary" size="sm" disabled={busy || !next}>
               {busy ? t("settings.account.password.saving") : t("settings.account.password.save")}
-            </button>
+            </Button>
           </form>
         )}
       </div>