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.

.storybook/main.ts

@@ -0,0 +1,35 @@
+import path from 'path'
+import { fileURLToPath } from 'url'
+import type { StorybookConfig } from '@storybook/nextjs-vite'
+
+const dirname = path.dirname(fileURLToPath(import.meta.url))
+
+const config: StorybookConfig = {
+  stories: [
+    '../src/**/*.mdx',
+    '../src/**/*.stories.@(js|jsx|mjs|ts|tsx)',
+  ],
+  addons: [
+    '@chromatic-com/storybook',
+    '@storybook/addon-vitest',
+    '@storybook/addon-a11y',
+    '@storybook/addon-docs',
+  ],
+  framework: '@storybook/nextjs-vite',
+  staticDirs: ['../public'],
+  viteFinal: async (config) => {
+    config.resolve ??= {}
+    config.resolve.alias = {
+      ...(config.resolve.alias as Record<string, string>),
+      '$shared': path.resolve(dirname, '../src/shared'),
+      '$entities': path.resolve(dirname, '../src/entities'),
+      '$features': path.resolve(dirname, '../src/features'),
+      '$widgets': path.resolve(dirname, '../src/widgets'),
+      '$app': path.resolve(dirname, '../src/app'),
+      '$routes': path.resolve(dirname, '../src/routes'),
+    }
+    return config
+  },
+}
+
+export default config