feat(ui/alerts): Alerts sidebar section with Inbox/All/Rules/Silences/History

Adds `buildAlertsTreeNodes` to sidebar-utils and renders an Alerts section
between Applications and Starred in LayoutShell. The section uses an
accordion pattern — entering `/alerts/*` collapses apps/admin/starred and
restores their state on leave.

gitnexus_impact(LayoutContent, upstream) = LOW (0 direct callers; rendered
only by LayoutShell's provider wrapper).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/components/sidebar-utils.test.ts

@@ -0,0 +1,17 @@
+import { describe, it, expect } from 'vitest';
+import { buildAlertsTreeNodes } from './sidebar-utils';
+
+describe('buildAlertsTreeNodes', () => {
+  it('returns 5 entries with inbox/all/rules/silences/history paths', () => {
+    const nodes = buildAlertsTreeNodes();
+    expect(nodes).toHaveLength(5);
+    const paths = nodes.map((n) => n.path);
+    expect(paths).toEqual([
+      '/alerts/inbox',
+      '/alerts/all',
+      '/alerts/rules',
+      '/alerts/silences',
+      '/alerts/history',
+    ]);
+  });
+});