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/ProfileEditor.tsx

@@ -7,6 +7,7 @@ import Modal from "./Modal";
 import { useConfirm } from "./ConfirmProvider";
 import { api, type DownloadProfile, type DownloadSpec } from "../lib/api";
 import { inputCls } from "./ui/form";
+import { Button } from "./ui/Button";
 
 const HEIGHTS = [null, 2160, 1440, 1080, 720, 480, 360];
 
@@ -145,12 +146,9 @@ export default function ProfileEditor({ onClose }: { onClose: () => void }) {
       </div>
 
       {!formOpen ? (
-        <button
-          onClick={startNew}
-          className="inline-flex items-center gap-1.5 px-3 py-1.5 rounded-lg text-sm font-medium btn-primary transition"
-        >
-          <Plus className="w-4 h-4" /> {t("downloads.profiles.new")}
-        </button>
+        <Button variant="primary" size="sm" icon={<Plus className="w-4 h-4" />} onClick={startNew}>
+          {t("downloads.profiles.new")}
+        </Button>
       ) : (
         <div className="rounded-xl border border-border p-4 space-y-3">
           <div>
@@ -263,19 +261,17 @@ export default function ProfileEditor({ onClose }: { onClose: () => void }) {
             />
           </div>
           <div className="flex justify-end gap-2 pt-1">
-            <button
-              onClick={() => setFormOpen(false)}
-              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={() => setFormOpen(false)}>
               {t("downloads.actions.cancel")}
-            </button>
-            <button
+            </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"
             >
               {editing ? t("downloads.profiles.save") : t("downloads.profiles.create")}
-            </button>
+            </Button>
           </div>
         </div>
       )}