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/gsmtap.py

@@ -81,7 +81,7 @@ class GsmtapReceiver:
         """
         try:
             packet, addr = self._sock.recvfrom(65536)
-        except socket.timeout:
+        except (socket.timeout, OSError):
             return None, None
 
         if len(packet) < _GSMTAP_HDR_SIZE: