[MISC] Support empty interactive scene with 'gs launch'.

[duburcqa]

gs launch now accepts no file argument: it opens an empty interactive scene where entities can be added live through the overlay's "Add Entity / Stage" panel and applied with "Rebuild Scene".

• filename is now optional (nargs="?", defaults to None).
• When omitted, no entity is loaded; the scene builds empty and starts paused, ready for interactive editing.
• Behavior with a file argument is unchanged.