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.tsx

@@ -0,0 +1,34 @@
+'use client'
+
+import { Button } from '$shared/ui'
+
+/**
+ * Fixed bottom utility bar with contact info and CV download.
+ */
+export function UtilityBar() {
+  /**
+   * Handles CV download action.
+   */
+  function handleDownloadCV() {
+    console.log('Downloading CV...')
+  }
+
+  return (
+    <div className="fixed bottom-0 left-0 right-0 bg-ochre-clay brutal-border-top z-40">
+      <div className="max-w-[2560px] mx-auto px-6 md:px-12 lg:px-16 py-4 flex items-center justify-between">
+        <div className="flex items-center gap-4">
+          <span className="text-sm uppercase tracking-wider">Contact</span>
+          <a
+            href="mailto:hello@allmy.work"
+            className="text-base hover:text-burnt-oxide transition-colors"
+          >
+            hello@allmy.work
+          </a>
+        </div>
+        <Button variant="primary" size="sm" onClick={handleDownloadCV}>
+          Download CV
+        </Button>
+      </div>
+    </div>
+  )
+}