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/__main__.py

@@ -13,6 +13,7 @@ from http.server import HTTPServer
 from .server import RequestHandler
 from .database import Database, DEFAULT_DB_PATH
 from .capture import CaptureManager, GsmtapListener, DirectSniffer
+from .. import __version__
 
 
 def _default_web_dir():
@@ -56,7 +57,7 @@ def main():
     server = HTTPServer((args.host, args.port), RequestHandler)
     server.web_dir = args.web_dir
 
-    print(f'simtrace2-pysniff-server — http://{args.host}:{args.port}', file=sys.stderr)
+    print(f'simtrace2-pysniff-server v{__version__} — http://{args.host}:{args.port}', file=sys.stderr)
     print(f'  capture mode: {args.capture}', file=sys.stderr)
     if args.capture == 'gsmtap':
         print(f'  GSMTAP port:  {args.gsmtap_port}', file=sys.stderr)

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):

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,