♻️(frontend) extract closeSidePanel action at the store level

Extract the logic that closes the side panel into a utility function
declared at the store module level, as recommended by Valtio.

This avoids re-creating the function on every render and prevents
extra re-renders in components that use it.

src/frontend/src/features/rooms/livekit/components/SidePanel.tsx

@@ -1,4 +1,4 @@
-import { layoutStore } from '@/stores/layout'
+import { closeSidePanel, layoutStore } from '@/stores/layout'
 import { css } from '@/styled-system/css'
 import { Heading } from 'react-aria-components'
 import { text } from '@/primitives/Text'
@@ -199,10 +199,7 @@ export const SidePanel = () => {
       ref={asideRef}
       title={title}
       ariaLabel={t('ariaLabel', { title })}
-      onClose={() => {
-        layoutStore.activePanelId = null
-        layoutStore.activeSubPanelId = null
-      }}
+      onClose={closeSidePanel}
       closeButtonTooltip={t('closeButton', {
         content: t(`content.${activeSubPanelId || activePanelId}`),
       })}

src/frontend/src/stores/layout.ts

@@ -34,3 +34,8 @@ export const setPinnedTrack = (trackRef: TrackReferenceOrPlaceholder): void => {
 export const clearPinnedTrack = (): void => {
   layoutStore.pinnedTrackRef = undefined
 }
+
+export const closeSidePanel = (): void => {
+  layoutStore.activePanelId = null
+  layoutStore.activeSubPanelId = null
+}