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/capture.py

@@ -94,12 +94,17 @@ class CaptureManager:
 
     def stop_session(self):
         if self._session_id is None:
-            return
+            return None
         self._backend.stop()
-        self._db.close_session(self._session_id)
+        if self._thread is not None:
+            self._thread.join(timeout=2.0)
         sid = self._session_id
         self._session_id = None
         self._start_time = 0.0
+        if self._db.count_messages(sid) == 0:
+            self._db.delete_session(sid)
+            return None
+        self._db.close_session(sid)
         return sid
 
     def _capture_loop(self):