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/UtilityBar.test.tsx

@@ -0,0 +1,30 @@
+import { describe, it, expect } from 'vitest'
+import { render, screen } from '@testing-library/react'
+import { UtilityBar } from './UtilityBar'
+
+describe('UtilityBar', () => {
+  describe('rendering', () => {
+    it('renders "Contact" label', () => {
+      render(<UtilityBar />)
+      expect(screen.getByText('Contact')).toBeInTheDocument()
+    })
+
+    it('renders email link with correct href', () => {
+      render(<UtilityBar />)
+      const link = screen.getByRole('link', { name: 'hello@allmy.work' })
+      expect(link).toBeInTheDocument()
+      expect(link).toHaveAttribute('href', 'mailto:hello@allmy.work')
+    })
+
+    it('renders "Download CV" button', () => {
+      render(<UtilityBar />)
+      expect(screen.getByRole('button', { name: /download cv/i })).toBeInTheDocument()
+    })
+
+    it('Download CV button has primary variant class', () => {
+      render(<UtilityBar />)
+      const btn = screen.getByRole('button', { name: /download cv/i })
+      expect(btn).toHaveClass('bg-burnt-oxide')
+    })
+  })
+})