The main and Settings windows were created at startup and kept alive hidden on
close, so an idle tray held two webview processes for surfaces the user may
never open. Both are now built on first show and destroyed on close, which
takes the idle footprint on macOS from ~160 MB to ~74 MB.
The WindowManager owns creation: it rebuilds the main window on the next show
and hands out live pointers, since a stored one goes stale. Every show is
deferred until the frontend reports it has rendered, so a freshly created
window is never on screen empty, with a timeout so a frontend that never
reports cannot strand a window hidden.
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