fix: open an approval window while another one has focus (closes #290)

The background centred each approval window on the last focused window,
which could be an earlier approval window still open. Headless Chrome
reports one as 1280x720, so the position came out where the browser refused
to create the window ("Bounds must be at least 50% within visible screen
space"), and the request failed with -32603 and no window. It now centres on
the last focused browser window.

Under load a test in the Chrome suite raised its prompt while the previous
test's window was still closing, and either hit that refusal or took the
closing window for its own. The runner now closes every approval window
between tests.

Model: opus-5-5

tests/coldWorkerChainId.test.js

@@ -113,7 +113,7 @@ function loadColdWorker(networkId, opts) {
             lastError: null,
         },
         windows: {
-            getLastFocused: (cb) => cb(null),
+            getLastFocused: (queryOptions, cb) => cb(null),
             create: (options, cb) => cb({ id: 1 }),
             remove: (id, cb) => {
                 if (cb) cb();