Show status in trace empty-state; discard empty captures; polish capture-mode

Frontend:
- trace empty-state now shows connection status + capture mode (boxed,
  colored) in the center, updated live via checkConnection
- capture-mode uses amber warning style (gsmtap/disabled), green for direct
- stopCapture clears currentSessionId

Server:
- gsmtap read_packet catches OSError on socket close (no more EBADF
  traceback on stop)
- stop_session joins the loop and discards a session with zero messages;
  /api/capture/stop reports discarded
- startup prints server version

Tests: 122 (gsmtap close, empty/non-empty capture lifecycle).

tests/test_gsmtap.py

@@ -0,0 +1,16 @@
+"""Tests for GSMTAP receiver shutdown behavior."""
+
+import unittest
+
+from simtrace2_pysniff.gsmtap import GsmtapReceiver
+
+
+class TestGsmtapReceiver(unittest.TestCase):
+    def test_read_after_close_returns_none(self):
+        r = GsmtapReceiver(bind_port=0)
+        r.close()
+        self.assertEqual(r.read_packet(), (None, None))
+
+
+if __name__ == '__main__':
+    unittest.main()