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.

eslint.config.mjs

@@ -1,3 +1,6 @@
+// For more info, see https://github.com/storybookjs/eslint-plugin-storybook#configuration-flat-config-format
+import storybook from "eslint-plugin-storybook";
+
 import { defineConfig, globalIgnores } from "eslint/config";
 import nextVitals from "eslint-config-next/core-web-vitals";
 import nextTs from "eslint-config-next/typescript";
@@ -13,6 +16,7 @@ const eslintConfig = defineConfig([
     "build/**",
     "next-env.d.ts",
   ]),
+  ...storybook.configs["flat/recommended"]
 ]);
 
 export default eslintConfig;