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/model/types.ts

@@ -0,0 +1,14 @@
+export type NavItem = {
+  /**
+   * Section HTML id for anchor scrolling
+   */
+  id: string
+  /**
+   * Display label
+   */
+  label: string
+  /**
+   * Display number prefix (e.g. "01")
+   */
+  number: string
+}