🐛(frontend) cache supportsBackgroundProcessors WebGL2 probe result

`supportsBackgroundProcessors()` creates a live WebGL2 context on
every call and never releases it. The method is called from render
paths (e.g. the Effects button on the join screen), so without
caching, each re-render leaks a context until the browser hits its
live-context limit.

This is one of the ways MediaPipe later fails with:

  "emscripten_webgl_create_context() returned error 0"

Support cannot change within a session, so probe once and cache the
result.

src/frontend/src/features/rooms/livekit/components/blur/index.ts

@@ -32,14 +32,18 @@ export interface BackgroundProcessorInterface extends TrackProcessor<Track.Kind>
 }
 
 export class BackgroundProcessorFactory {
+  private static _isSupported?: boolean
+
   static hasModernApiSupport() {
     return ProcessorWrapper.hasModernApiSupport
   }
 
   static isSupported() {
-    return (
-      supportsBackgroundProcessors() || BackgroundCustomProcessor.isSupported
-    )
+    if (this._isSupported === undefined) {
+      this._isSupported =
+        supportsBackgroundProcessors() || BackgroundCustomProcessor.isSupported
+    }
+    return this._isSupported
   }
 
   static getProcessor(