✨(frontend) introduce basic document picture-in-picture hook

Add a hook that manages the logic needed to open a document
picture-in-picture window, stores a ref to the window in a global
store, and—once ready—mounts a portal to duplicate content into
the PiP document's container.

Mounting the PiP content via a portal lets us share the same React
tree as the main app, and therefore share application state. This
comes with some trade-offs, particularly around components that
rely on the DOM hierarchy: react-aria popovers and overlays may
not behave correctly across documents.

The logic is kept to a minimum here. This first commit does nothing
more than open a window containing a loading spinner. The topic is
new to us, so plenty is likely to be refined in follow-ups.

Co-authored-by: Cyril <c.gromoff@gmail.com>

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to
 - ✨(frontend) add synchronizer for room metadata updates
 - ✨(frontend) make reaction toolbar responsive on small viewports
 - ✨(frontend) enable reactions on mobile devices
+- ✨(frontend) introduce picture-in-picture meeting
 
 ### Changed