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/tests/test_web.py

@@ -225,6 +225,26 @@ class TestReconnect(unittest.TestCase):
         fake.close()
 
 
+class TestQuit(unittest.TestCase):
+    def test_quit_closes_the_page_and_rests_the_motors(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            lib = make_library(tmp)
+            fake, c = make_controller(100)
+            s = socket.socket(); s.bind(('127.0.0.1', 0)); port = s.getsockname()[1]; s.close()
+            th = threading.Thread(target=serve, args=(c, CFG), kwargs={'port': port, 'open_browser': False, 'library': lib},
+                                  daemon=True)
+            th.start()
+            from sandtable import launcher
+            self.assertTrue(wait_for(lambda: launcher.status(port) is not None, 10))
+            self.assertTrue(launcher.status(port)['can_quit'])
+            launcher.quit_gyre(port)
+            th.join(10)
+            self.assertFalse(th.is_alive())                  # the page has closed
+            self.assertIsNone(launcher.status(port))
+            self.assertTrue(fake.sleeping)                   # motors off
+            fake.close()
+
+
 class TestHttp(unittest.TestCase):
     @classmethod
     def setUpClass(cls):