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

frontend/tests/comprion.test.js

@@ -498,3 +498,29 @@ test('Change details modal: header, summary, legend and both views', () => {
 	assert.match(el('file-diff-sub').textContent, /Record 2 \/ 5/);
 	assert.match(el('file-diff-body').innerHTML, /image is frozen/);
 });
+
+// ── Session loading must survive "no active capture" ───────────────────
+eval(extractFunc(src, 'stopCaptureIfRunning'));
+
+test('stopCaptureIfRunning tolerates "no active capture" (400)', async () => {
+	let calls = 0;
+	API.stopCapture = async () => {
+		calls++;
+		throw Object.assign(new Error('No active capture'), {status: 400});
+	};
+	await stopCaptureIfRunning();          // must resolve, not reject
+	assert.equal(calls, 1);
+	API.stopCapture = async () => { calls++; };
+	await stopCaptureIfRunning();
+	assert.equal(calls, 2);
+});
+
+test('loadSession stops a capture tolerantly', () => {
+	const body = extractFunc(src, 'loadSession');
+	assert.match(body, /await stopCaptureIfRunning\(\)/);
+	assert.doesNotMatch(body, /await API\.stopCapture\(\)/);
+	// The capture button also resets its state after a tolerated stop.
+	const stop = extractFunc(src, 'stopCapture');
+	assert.match(stop, /await stopCaptureIfRunning\(\)/);
+	assert.match(stop, /STATE\.captureActive = false;/);
+});