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/WatchPage.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useRef, useState } from "react";
 import { Download, Link2, Lock, PlayCircle } from "lucide-react";
+import { Button } from "./ui/Button";
 
 // Standalone, login-free video page for a shared /watch/<token> link. Rendered OUTSIDE the app
 // tree (no auth, no providers) — like the /privacy and /terms pages. Uses plain fetch and a tiny
@@ -200,13 +201,16 @@ export default function WatchPage() {
                 className="w-full rounded-lg bg-surface border border-border px-3 py-2 text-sm outline-hidden focus:ring-2 focus:ring-accent/40"
               />
               {error && <div className="text-sm text-danger mt-2">{error}</div>}
-              <button
+              <Button
+                variant="primary"
+                size="md"
+                fullWidth
                 onClick={unlock}
                 disabled={!password || unlocking}
-                className="mt-4 w-full rounded-lg btn-primary disabled:opacity-50 py-2 text-sm font-medium transition"
+                className="mt-4"
               >
                 {T.watch}
-              </button>
+              </Button>
             </div>
           </div>
         )}