fix(embedded-mpv): do not restart the stream on every volume change

Same root cause as the loading-loop fix: the session-creation effect
read this.volume() while building startSession's initialVolume argument,
which made volume a tracked dependency of the effect. Each volume tick
re-ran the effect, the cleanup disposed the active session, and a fresh
one was created — which for VOD/series meant restarting playback from
the beginning.

Read volume via untracked() inside the effect. The value is only needed
once at session creation; subsequent volume changes flow through
controller.applyVolume() and never go near the effect.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Entire-Checkpoint: f957cd9849e0

libs/ui/playback/src/lib/embedded-mpv-player/embedded-mpv-player.component.ts

@@ -10,6 +10,7 @@ import {
     inject,
     input,
     signal,
+    untracked,
     viewChild,
 } from '@angular/core';
 import { MatButtonModule } from '@angular/material/button';
@@ -271,10 +272,14 @@ export class EmbeddedMpvPlayerComponent implements OnDestroy {
                 return;
             }
 
+            // volume is read untracked so adjusting it during playback does
+            // not re-trigger this effect (which would tear down and recreate
+            // the session, restarting the stream from the beginning).
+            // Subsequent volume changes flow through controller.applyVolume.
             const teardown = this.controller.startSession(
                 viewport.nativeElement,
                 playback,
-                this.volume()
+                untracked(() => this.volume())
             );
             onCleanup(teardown);
         });