fix(player): silence active media on secondary-device guard install

frontend/angular/src/app/features/player/player-shell.component.ts

@@ -24,6 +24,11 @@ type MediaPrototypeWithGuardState = {
   };
 };
 
+type GuardableMediaElement = {
+  muted?: boolean;
+  pause?: () => void;
+};
+
 function resolveLocalStorage(): Storage | undefined {
   if (typeof window === 'undefined') {
     return undefined;
@@ -202,6 +207,8 @@ export class PlayerShellComponent implements OnInit, OnDestroy {
       return;
     }
 
+    this.silenceExistingMediaElements();
+
     const mediaPrototype = (window as Window & { HTMLMediaElement?: { prototype?: MediaPrototypeWithGuardState } }).HTMLMediaElement
       ?.prototype;
 
@@ -235,6 +242,26 @@ export class PlayerShellComponent implements OnInit, OnDestroy {
     };
   }
 
+  private silenceExistingMediaElements(): void {
+    if (typeof document === 'undefined' || typeof document.querySelectorAll !== 'function') {
+      return;
+    }
+
+    const activeElements = document.querySelectorAll('audio,video') as
+      | NodeListOf<GuardableMediaElement>
+      | GuardableMediaElement[]
+      | undefined;
+
+    if (!activeElements || typeof (activeElements as { forEach?: unknown }).forEach !== 'function') {
+      return;
+    }
+
+    activeElements.forEach((element) => {
+      element.muted = true;
+      element.pause?.();
+    });
+  }
+
   private scheduleStateSync(): void {
     this.clearStateSyncTimer();