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.

vitest.config.ts

@@ -8,6 +8,7 @@ export default defineConfig({
     environment: 'jsdom',
     globals: true,
     setupFiles: ['./src/test/setup.ts'],
+    exclude: ['**/*.stories.tsx', 'node_modules/**'],
   },
   resolve: {
     alias: {