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/ConfirmProvider.tsx

@@ -1,6 +1,7 @@
 import { createContext, useCallback, useContext, useState, type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
 import Modal from "./Modal";
+import { Button } from "./ui/Button";
 
 // App-styled, promise-based replacement for window.confirm. Wrap the tree in
 // <ConfirmProvider> once, then anywhere:  const confirm = useConfirm();
@@ -100,26 +101,25 @@ export function ConfirmProvider({ children }: { children: ReactNode }) {
           <div
             className={multi ? "flex flex-col gap-2 mt-5" : "flex justify-end gap-2 mt-5 flex-wrap"}
           >
-            <button
+            <Button
+              variant="outline"
               autoFocus={anyDanger}
               onClick={() => close(null)}
-              className={`px-4 py-2 rounded-lg text-sm border border-border text-muted hover:text-fg hover:bg-surface transition ${
-                multi ? "w-full order-last" : ""
-              }`}
+              fullWidth={multi}
+              className={multi ? "order-last" : ""}
             >
               {state.opts.cancelLabel ?? t("common.cancel")}
-            </button>
+            </Button>
             {choices.map((c, i) => (
-              <button
+              <Button
                 key={c.id}
+                variant={c.danger ? "danger" : "primary"}
                 autoFocus={!anyDanger && i === choices.length - 1}
                 onClick={() => close(c.id)}
-                className={`px-4 py-2 rounded-lg text-sm font-semibold transition ${
-                  multi ? "w-full" : ""
-                } ${c.danger ? "btn-danger" : "btn-primary"}`}
+                fullWidth={multi}
               >
                 {c.label || t("common.confirm")}
-              </button>
+              </Button>
             ))}
           </div>
         </Modal>