🩹(frontend) avoid video glitch on the prejoin while init a processor

We've introduced simplifications that improve performance, enhance ux,
and contribute to the overall perception of experience quality.

Previously, our processor handling was overly complex. LiveKit allows us to set
a processor before starting the local video preview track, which eliminates
the black blink glitch that appeared when loading the join component
with a default processor.

This change prevents the unnecessary stopping and restarting
of the local video track.

I'm glad this issue is now resolved.

We also simplified component communication by avoiding props drilling.
Now, we use a single flag to indicate when the user is ready to enter the room.
This significantly reduces the complexity of props passed through components.

src/frontend/src/features/rooms/components/Conference.tsx

@@ -1,7 +1,10 @@
 import { useEffect, useMemo, useState } from 'react'
 import { useQuery } from '@tanstack/react-query'
 import { useTranslation } from 'react-i18next'
-import { LiveKitRoom } from '@livekit/components-react'
+import {
+  LiveKitRoom,
+  usePersistentUserChoices,
+} from '@livekit/components-react'
 import {
   DisconnectReason,
   MediaDeviceFailure,
@@ -31,18 +34,20 @@ import { isFireFox } from '@/utils/livekit'
 
 export const Conference = ({
   roomId,
-  userConfig,
   initialRoomData,
   mode = 'join',
 }: {
   roomId: string
-  userConfig: LocalUserChoices
   mode?: 'join' | 'create'
   initialRoomData?: ApiRoom
 }) => {
   const posthog = usePostHog()
   const { data: apiConfig } = useConfig()
 
+  const { userChoices: userConfig } = usePersistentUserChoices() as {
+    userChoices: LocalUserChoices
+  }
+
   useEffect(() => {
     posthog.capture('visit-room', { slug: roomId })
   }, [roomId, posthog])