Auto-wrap desktop session tabs to a second row on overflow

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).

src/web/public/styles.css

@@ -312,6 +312,13 @@ body {
   max-height: 120px;
 }
 
+.session-tabs.tabs-auto-wrap {
+  flex-wrap: wrap;
+  overflow-x: hidden;
+  overflow-y: auto;
+  max-height: 96px;
+}
+
 .session-tabs::-webkit-scrollbar {
   width: 4px;
   height: 0;