fix(test): update E2E test for onboarding screen + increase timeouts

The U7 onboarding view replaced "No accounts yet." with "Welcome to
SharedInbox", causing the E2E test to spin for the full timeout budget
(pumping slowly in headless CI) before failing. Fix the finder and
bump per-attempt timeout from 240s → 360s and CI job ceiling from
20 min → 30 min to give the full account-add → send → verify flow
room to complete.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

stalwart-dev/integration_ui_test.sh

@@ -139,7 +139,7 @@ done
 _e2e_exit=0
 for _attempt in 1 2; do
     ts "E2E attempt $_attempt (DISPLAY=$DISPLAY)"
-    timeout 240 fvm flutter test integration_test/ -d linux
+    timeout 360 fvm flutter test integration_test/ -d linux
     _e2e_exit=$?
     [ $_e2e_exit -eq 0 ] && break
     if [ $_attempt -lt 2 ]; then