Add createInstanceWindow() — a full-chrome peer of the primary that
renders the complete app (sidebar, routing, its own draft) against the
shared backend, so several GUI windows can run at once. Mirrors the
primary's window options + chatWindowWebPreferences (backgroundThrottling
stays off so a streamed answer never stalls when blurred) but never
overwrites the mainWindow global and doesn't respawn the backend — the
renderer's getConnection() joins the running one. New windows cascade off
their source via the pure, tested instanceWindowBounds().
Exposed via the hermes🪟openInstance IPC and a "New Window" File
menu item. Per-window fullscreen state now targets the window itself, and
titlebar/native-theme repaints reach every open chat window instead of
only the primary.
Retires the now-orphaned compact new-session pop-out (its only caller was
⌘⇧N, repointed in the follow-up commit): drops createNewSessionWindow,
the hermes🪟openNewSession handler, and the newSession/new=1 URL
flag.
