feat: add Storybook with component stories

Installs @storybook/nextjs-vite. Stories co-located with components,
grouped by layer (Shared/Entities/Widgets). Multi-variant cases use
render functions instead of one story per variant/size.

src/widgets/Navigation/ui/MobileNav.stories.tsx

@@ -0,0 +1,28 @@
+import type { Meta, StoryObj } from '@storybook/nextjs-vite'
+import { MobileNav } from './MobileNav'
+
+// MobileNav is lg:hidden — it renders only on mobile viewports.
+// Use the viewport toolbar in Storybook to switch to a mobile size to see it.
+const meta: Meta<typeof MobileNav> = {
+  title: 'Widgets/MobileNav',
+  component: MobileNav,
+  parameters: {
+    viewport: {
+      defaultViewport: 'mobile1',
+    },
+  },
+}
+
+export default meta
+
+type Story = StoryObj<typeof MobileNav>
+
+export const Default: Story = {
+  args: {
+    items: [
+      { id: 'bio', label: 'Bio', number: '01' },
+      { id: 'work', label: 'Work', number: '02' },
+      { id: 'contact', label: 'Contact', number: '03' },
+    ],
+  },
+}