Start Gyre from an icon: launch / quit / install-desktop

- python3 -m sandtable install-desktop puts a Gyre icon in the app grid and on
  the desktop (freedesktop .desktop entry with a "Quit Gyre" action).
- The icon runs `launch`: starts the web page in the background (no terminal,
  own session, log in .cache/server.log) if it isn't running, then opens it.
  A dialog shows the reason if it can't start.
- Quit link on the page (local computer only) and `quit`: stop any design,
  motors off, close the page.
- Gyre icon (sandtable/static/gyre.svg), also the page's tab icon.
- Simulator takes several lines per tick, so streaming tests aren't slowed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

software/sandtable/static/gyre.svg

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