fix: wrap isElectron in computed for proper reactivity

Changed from direct signal assignment to computed() wrapper to ensure
consistent reactive pattern and clearer intent.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/src/app/xtream/vod-details/vod-details.component.ts

@@ -87,7 +87,7 @@ export class VodDetailsComponent implements OnDestroy {
     // ============ Computed State ============
 
     /** Whether running in Electron (downloads available) */
-    readonly isElectron = this.downloadsService.isAvailable;
+    readonly isElectron = computed(() => this.downloadsService.isAvailable());
 
     /** Normalized metadata for display */
     readonly normalizedMeta = computed(() => {