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).

test/tab-overflow.test.ts

@@ -0,0 +1,51 @@
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+import vm from 'node:vm';
+import { describe, expect, it } from 'vitest';
+
+function loadTabOverflowHelper() {
+  const context = vm.createContext({ window: {}, globalThis: {} });
+  const source = readFileSync(resolve(import.meta.dirname, '../src/web/public/constants.js'), 'utf8');
+  vm.runInContext(source, context, { filename: 'constants.js' });
+  return (context.window as { CodemanTabOverflow: { shouldAutoWrapTabs: (input: unknown) => boolean } })
+    .CodemanTabOverflow;
+}
+
+describe('tab overflow layout policy', () => {
+  it('auto-wraps desktop tabs when their rendered width exceeds available tab space', () => {
+    const helper = loadTabOverflowHelper();
+
+    expect(
+      helper.shouldAutoWrapTabs({
+        deviceType: 'desktop',
+        manualTwoRows: false,
+        tabCount: 18,
+        scrollWidth: 1400,
+        clientWidth: 760,
+      })
+    ).toBe(true);
+  });
+
+  it('does not auto-wrap when manual tall tabs are enabled or on mobile/tablet', () => {
+    const helper = loadTabOverflowHelper();
+
+    expect(
+      helper.shouldAutoWrapTabs({
+        deviceType: 'desktop',
+        manualTwoRows: true,
+        tabCount: 18,
+        scrollWidth: 1400,
+        clientWidth: 760,
+      })
+    ).toBe(false);
+    expect(
+      helper.shouldAutoWrapTabs({
+        deviceType: 'mobile',
+        manualTwoRows: false,
+        tabCount: 18,
+        scrollWidth: 1400,
+        clientWidth: 320,
+      })
+    ).toBe(false);
+  });
+});