feat(chat): show chat in sidebar with opened Viewer

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Feature request

When opening a Viewer from Talk app (shared file), chat is hidden behind the dialog mask, but sidebar is still available

Current	Proposal

Implementation

• Viewer state is available from composable: https://github.com/nextcloud/spreed/blob/edc1324ba7b12425e1f50f1657879304f66dbd2f/src/composables/useViewer.js#L131
• Chat component in MainView should somehow unmounted / teleported / blocked, so there's only single instance of chat, and no duplicate logic triggered by scrolling / messages list update / e.t.c.
• Sidebar should be force triggered to open, so chat is visible

Start with src/composables/useViewer.js at the referenced viewer-state entry point, then inspect the chat component in MainView and the sidebar-opening flow. Done means opening a Viewer from Talk leaves one chat instance visible in the sidebar without duplicate logic triggered by scrolling or message-list updates.