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/MobileNav.test.tsx

@@ -0,0 +1,46 @@
+import { describe, it, expect, vi } from 'vitest'
+import { render, screen } from '@testing-library/react'
+import userEvent from '@testing-library/user-event'
+import { MobileNav } from './MobileNav'
+import type { NavItem } from '../model/types'
+
+const ITEMS: NavItem[] = [{ id: 'about', label: 'About', number: '01' }]
+
+describe('MobileNav', () => {
+  describe('rendering', () => {
+    it('renders title "allmy.work"', () => {
+      render(<MobileNav items={ITEMS} />)
+      expect(screen.getByText('allmy.work')).toBeInTheDocument()
+    })
+
+    it('renders toggle button with text "Menu" initially', () => {
+      render(<MobileNav items={ITEMS} />)
+      expect(screen.getByRole('button', { name: 'Menu' })).toBeInTheDocument()
+    })
+
+    it('menu items are hidden initially', () => {
+      render(<MobileNav items={ITEMS} />)
+      expect(screen.queryByRole('button', { name: /about/i })).not.toBeInTheDocument()
+    })
+  })
+
+  describe('interactions', () => {
+    it('click toggle shows item buttons and changes label to "Close"', async () => {
+      render(<MobileNav items={ITEMS} />)
+      await userEvent.click(screen.getByRole('button', { name: 'Menu' }))
+      expect(screen.getByRole('button', { name: 'Close' })).toBeInTheDocument()
+      expect(screen.getByText('About')).toBeInTheDocument()
+    })
+
+    it('click item button closes the menu', async () => {
+      render(<MobileNav items={ITEMS} />)
+      await userEvent.click(screen.getByRole('button', { name: 'Menu' }))
+      // item button label contains number + label text; find by accessible name fragment
+      const itemBtn = screen.getAllByRole('button').find(b => b.textContent?.includes('About'))
+      expect(itemBtn).toBeDefined()
+      await userEvent.click(itemBtn!)
+      expect(screen.queryByText('Close')).not.toBeInTheDocument()
+      expect(screen.getByRole('button', { name: 'Menu' })).toBeInTheDocument()
+    })
+  })
+})