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_capture.py

@@ -0,0 +1,49 @@
+"""Tests for CaptureManager session lifecycle (empty-session discard)."""
+
+import os
+import tempfile
+import unittest
+
+from simtrace2_pysniff.server.capture import CaptureManager
+from simtrace2_pysniff.server.database import Database
+
+
+class _FakeBackend:
+    def __init__(self, messages=()):
+        self._messages = list(messages)
+
+    def start(self):
+        pass
+
+    def stop(self):
+        pass
+
+    def iter_messages(self):
+        yield from self._messages
+
+
+class TestCaptureManager(unittest.TestCase):
+    def _manager(self, backend):
+        tmp = tempfile.NamedTemporaryFile(suffix='.db', delete=False)
+        tmp.close()
+        db = Database(tmp.name)
+        self.addCleanup(lambda: (db._conn.close(), os.remove(tmp.name)))
+        return CaptureManager(backend, db), db
+
+    def test_empty_capture_discarded(self):
+        mgr, db = self._manager(_FakeBackend())
+        sid = mgr.start_session()
+        self.assertIsNotNone(sid)
+        self.assertIsNone(mgr.stop_session())
+        self.assertIsNone(db.get_session(sid))
+
+    def test_non_empty_capture_kept(self):
+        mgr, db = self._manager(_FakeBackend([('tpdu', b'\x80\xf2\x00\x00\x00', 0)]))
+        sid = mgr.start_session()
+        self.assertEqual(mgr.stop_session(), sid)
+        self.assertIsNotNone(db.get_session(sid))
+        self.assertEqual(db.count_messages(sid), 1)
+
+
+if __name__ == '__main__':
+    unittest.main()

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()