Fix session loading: a no-op capture stop aborted the load (v2.4.1)

- `loadSession` begins by stopping a running capture; when there is nothing
  to stop the server answers 400 'No active capture' (or 403 with capture
  disabled) — and since the v2.3.1 `_fetch` status fix those answers throw, so
  the whole load was skipped and the UI alerted "Failed to load session":
  no session could be opened at all
- new `stopCaptureIfRunning()` swallows exactly that no-op answer; both
  `loadSession` and the capture button use it, and the button now resets its
  state outside the try so a tolerated stop still clears the UI
- the `_fetch` status check stays — real errors keep surfacing (an unknown
  session still throws 'Session not found')
- tests: the helper resolves on a 400 and still calls once; source pins that
  `loadSession` uses the tolerant helper (the regression guard) and that
  `stopCapture` resets the state
- v2.4.1; SW cache simtrace-analyser-v66

tests/test_pcap_import.py

@@ -224,7 +224,7 @@ class TestImportEndpoint(unittest.TestCase):
 class TestVersion(unittest.TestCase):
     def test_package_version(self):
         from simtrace2_pysniff import __version__
-        self.assertEqual(__version__, '2.4.0')
+        self.assertEqual(__version__, '2.4.1')
 
     def test_status_includes_version(self):
         import os