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)