✨(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>

src/frontend/src/locales/en/rooms.json

@@ -241,6 +241,10 @@
       "username": "Update Your Name",
       "effects": "Backgrounds and Effects",
       "switchCamera": "Switch camera",
+      "pictureInPicture": {
+        "enter": "Picture-in-picture",
+        "exit": "Close picture-in-picture"
+      },
       "fullscreen": {
         "enter": "Fullscreen",
         "exit": "Exit fullscreen mode"