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>

BUILD.md

@@ -247,17 +247,15 @@ The full details are in [`software/README.md`](software/README.md). In short, fr
 5. **Glass:** lay it on the sand ring. The two finger notches in the rim help lift it out.
 
 ## 14. Everyday use
-From the `software` folder, run:
-```
-python3 -m sandtable web
-```
-This opens a page in your browser with the design library:
+Once, from the `software` folder, run `python3 -m sandtable install-desktop`. It adds a **Gyre** icon to your app grid and desktop (Linux). On GNOME, right-click the desktop icon once and choose **Allow Launching**.
+
+From then on, click the Gyre icon. It opens a page in your browser with the design library:
 - click a design to draw it; it homes and erases the sand first;
 - **Play all** runs the whole list, shuffled and repeating;
 - when the designs finish (or you press Stop), the motors switch off and the page shows "Motors off". The next design homes first;
 - **Upload** takes `.thr` files (from [sandify.org](https://sandify.org)) and `.svg` line drawings.
 
-Leave the terminal open while the table runs. See [`software/README.md`](software/README.md) for everything else.
+**Quit**, at the bottom of the page, closes Gyre. See [`software/README.md`](software/README.md) for everything else, and [`pi/README.md`](pi/README.md) to run the table from a Raspberry Pi Zero 2 W with no computer at all.
 
 ## 15. Troubleshooting
 | Symptom | Likely cause | Fix |