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/downloadModals.tsx

@@ -6,6 +6,7 @@ import { Plus, Trash2 } from "lucide-react";
 import Modal from "./Modal";
 import { api, type DownloadJob } from "../lib/api";
 import { inputCls } from "./ui/form";
+import { Button } from "./ui/Button";
 import { IconBtn } from "./downloadRow";
 
 // Per-item user modals split out of DownloadCenter (self-contained, prop-driven): rename/metadata
@@ -101,13 +102,14 @@ export function MetaEditModal({ job, onClose }: { job: DownloadJob; onClose: ()
         </div>
         <p className="text-xs text-muted">{t("downloads.edit.hint")}</p>
         <div className="flex justify-end">
-          <button
+          <Button
+            variant="primary"
+            size="sm"
             onClick={() => save.mutate()}
             disabled={!name.trim() || save.isPending}
-            className="px-3 py-1.5 rounded-lg text-sm font-medium btn-primary disabled:opacity-50 transition"
           >
             {t("downloads.edit.save")}
-          </button>
+          </Button>
         </div>
       </div>
     </Modal>
@@ -158,22 +160,20 @@ export function ErrorModal({
         )}
         <div className="flex justify-end gap-2 pt-1">
           {retryable && (
-            <button
+            <Button
+              variant="primary"
+              size="sm"
               onClick={() => {
                 onRetry();
                 onClose();
               }}
-              className="px-3 py-1.5 rounded-lg text-sm font-medium btn-primary transition"
             >
               {t("downloads.actions.retry")}
-            </button>
+            </Button>
           )}
-          <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("common.close")}
-          </button>
+          </Button>
         </div>
       </div>
     </Modal>