♻️(frontend) extract getTrackKey to shared layout utils

Move track key helper out of pip so layouts can reuse it.

src/frontend/src/features/pip/components/layout/PipGridLayout.tsx

@@ -4,7 +4,7 @@ import { styled } from '@/styled-system/jsx'
 import { ParticipantTile } from '@/features/rooms/livekit/components/ParticipantTile'
 import { usePipElementSize } from '../../hooks/usePipElementSize'
 import { computePipGridLayout } from '../../utils/pipGrid'
-import { getTrackKey } from '../../utils/pipTrackSelection'
+import { getTrackKey } from '@/features/layout/utils/trackSelection'
 
 type PipGridLayoutProps = {
   tracks: TrackReferenceOrPlaceholder[]

src/frontend/src/features/pip/components/layout/PipScreenShareLayout.tsx

@@ -2,7 +2,7 @@ import { memo } from 'react'
 import type { TrackReferenceOrPlaceholder } from '@livekit/components-core'
 import { styled } from '@/styled-system/jsx'
 import { ParticipantTile } from '@/features/rooms/livekit/components/ParticipantTile'
-import { getTrackKey } from '../../utils/pipTrackSelection'
+import { getTrackKey } from '@/features/layout/utils/trackSelection'
 
 type PipScreenShareLayoutProps = {
   screenShareTrack: TrackReferenceOrPlaceholder