Server: a stop signal no longer crashes it (SIGABRT) while the app holds stdin

The --exit-on-eof watcher read stdin with a buffered read, which holds stdin's
lock. When SIGTERM stopped the server first, Python aborted at exit trying to
take that lock back, and the app showed "The server stopped unexpectedly
(SIGABRT)". It now uses os.read. The startup line is printed inside the try,
so a signal that arrives while it's printed still runs the cleanup.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/test_server.py

@@ -279,6 +279,18 @@ class OneServer(unittest.TestCase):
         r = conn.getresponse()
         self.assertEqual(r.status, 403, r.read())
 
+    @unittest.skipIf(os.name == "nt", "no SIGTERM on Windows")
+    def test_sigterm_while_the_app_holds_stdin_exits_cleanly(self):
+        """The app keeps stdin open; a stop signal used to abort Python (SIGABRT) at exit."""
+        env = {**os.environ, "FRAME_CONTROL_DATA_DIR": tempfile.mkdtemp(prefix="frame-one-server-"),
+               "FRAME_ALIAS": "frame-control-test.invalid"}
+        proc = subprocess.Popen([sys.executable, str(ROOT / "ui" / "server.py"), "--port", "0", "--exit-on-eof"],
+                                env=env, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, text=True)
+        self.addCleanup(lambda: (proc.stdin.close(), proc.stdout.close()))
+        self.assertIn("Frame Control on", proc.stdout.readline())
+        proc.terminate()
+        self.assertEqual(proc.wait(30), 0, proc.stdout.read())
+
 
 class ArtworkSettings(unittest.TestCase):
     """The settings panel's endpoints, with and without the page's X-Frame-UI key."""