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/shared/ui/Input/ui/Input.stories.tsx

@@ -0,0 +1,59 @@
+import type { Meta, StoryObj } from '@storybook/nextjs-vite'
+import { Input, Textarea } from './Input'
+
+const meta: Meta<typeof Input> = {
+  title: 'Shared/Input',
+  component: Input,
+  decorators: [
+    (Story) => (
+      <div className="p-8 bg-ochre-clay max-w-sm">
+        <Story />
+      </div>
+    ),
+  ],
+}
+
+export default meta
+
+type Story = StoryObj<typeof Input>
+
+export const Default: Story = {
+  args: {},
+}
+
+export const WithLabel: Story = {
+  args: {
+    label: 'Email address',
+  },
+}
+
+export const WithError: Story = {
+  args: {
+    label: 'Email',
+    error: 'This field is required',
+  },
+}
+
+export const WithPlaceholder: Story = {
+  args: {
+    placeholder: 'Enter your email',
+    type: 'email',
+  },
+}
+
+export const TextareaStory: Story = {
+  name: 'Textarea',
+  render: () => (
+    <div className="p-8 bg-ochre-clay max-w-sm">
+      <Textarea label="Message" rows={4} />
+    </div>
+  ),
+}
+
+export const TextareaWithError: Story = {
+  render: () => (
+    <div className="p-8 bg-ochre-clay max-w-sm">
+      <Textarea label="Message" error="Too short" rows={4} />
+    </div>
+  ),
+}