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

[Antreesy]

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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:

  spreed/src/composables/useViewer.js

  Line 131 in edc1324

  isViewerOpen,

• 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