When desktop session tabs overflow one row, wrap them to a second row instead
of horizontal scroll — unless the user has pinned the manual two-row layout
(tabTwoRows). Mobile/tablet keep horizontal scroll. The wrap policy
(shouldAutoWrapTabs) lives in constants.js as a pure, unit-testable function;
updateTabOverflowMode() measures overflow after each tab render and toggles
.tabs-auto-wrap.
Test: test/tab-overflow.test.ts (vm-loads constants.js, asserts the policy).
