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/ui.py

@@ -14,6 +14,7 @@ from .grbl import RT_HOLD, RT_JOG_CANCEL, GrblError
 PAGE = r"""<!doctype html>
 <html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width,initial-scale=1">
 <title>Gyre - Jog</title>
+<link rel="icon" href="/favicon.svg" type="image/svg+xml">
 <style>
 :root{--bg:#f6f4ef;--card:#fff;--ink:#222;--mute:#666;--line:#ddd;--acc:#2f6f5e;--stop:#c0392b}
 @media (prefers-color-scheme:dark){:root{--bg:#1b1c1e;--card:#26282b;--ink:#eee;--mute:#aaa;--line:#3a3d41;--acc:#5fb49c}}