feat: add MobileNav, SidebarNav, and UtilityBar widgets

TDD implementation of three navigation widgets: mobile overlay toggle,
fixed sidebar with IntersectionObserver-driven active section tracking,
and utility bar with contact info and CV download action.

src/widgets/Navigation/ui/SidebarNav.test.tsx

@@ -0,0 +1,62 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { render, screen } from '@testing-library/react'
+import { SidebarNav } from './SidebarNav'
+import type { NavItem } from '../model/types'
+
+const ITEMS: NavItem[] = [
+  { id: 'bio', label: 'Bio', number: '01' },
+  { id: 'work', label: 'Work', number: '02' },
+]
+
+beforeEach(() => {
+  global.IntersectionObserver = vi.fn(function () {
+    return {
+      observe: vi.fn(),
+      disconnect: vi.fn(),
+      unobserve: vi.fn(),
+    }
+  }) as unknown as typeof IntersectionObserver
+})
+
+describe('SidebarNav', () => {
+  describe('rendering', () => {
+    it('renders a nav element', () => {
+      render(<SidebarNav items={ITEMS} />)
+      expect(screen.getByRole('navigation')).toBeInTheDocument()
+    })
+
+    it('renders "Index" heading', () => {
+      render(<SidebarNav items={ITEMS} />)
+      expect(screen.getByText('Index')).toBeInTheDocument()
+    })
+
+    it('renders "Digital Monograph" subtitle', () => {
+      render(<SidebarNav items={ITEMS} />)
+      expect(screen.getByText('Digital Monograph')).toBeInTheDocument()
+    })
+
+    it('renders each item label and number', () => {
+      render(<SidebarNav items={ITEMS} />)
+      expect(screen.getByText('Bio')).toBeInTheDocument()
+      expect(screen.getByText('01')).toBeInTheDocument()
+      expect(screen.getByText('Work')).toBeInTheDocument()
+      expect(screen.getByText('02')).toBeInTheDocument()
+    })
+
+    it('renders "Quick Links" section', () => {
+      render(<SidebarNav items={ITEMS} />)
+      expect(screen.getByText('Quick Links')).toBeInTheDocument()
+    })
+
+    it('renders Email quick link', () => {
+      render(<SidebarNav items={ITEMS} />)
+      expect(screen.getByRole('link', { name: 'Email' })).toBeInTheDocument()
+    })
+
+    it('renders a button for each item', () => {
+      render(<SidebarNav items={ITEMS} />)
+      const buttons = screen.getAllByRole('button')
+      expect(buttons.length).toBeGreaterThanOrEqual(ITEMS.length)
+    })
+  })
+})