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).

simtrace2_pysniff/server/server.py

@@ -293,6 +293,9 @@ class RequestHandler(BaseHTTPRequestHandler):
             self._send_error(400, 'No active capture')
             return
         session_id = self.capture.stop_session()
+        if session_id is None:
+            self._send_json({'session_id': None, 'messages_count': 0, 'discarded': True})
+            return
         session = self.db.get_session(session_id)
         self._send_json({
             'session_id': session_id,