refactor(config): move storybook and vitest configs to config/, import app.css in storybook preview

config/storybook/main.ts

@@ -0,0 +1,17 @@
+import type { StorybookConfig } from '@storybook/sveltekit';
+
+const config: StorybookConfig = {
+  "stories": [
+    "../src/**/*.mdx",
+    "../src/**/*.stories.@(js|ts|svelte)"
+  ],
+  "addons": [
+    "@storybook/addon-svelte-csf",
+    "@chromatic-com/storybook",
+    "@storybook/addon-vitest",
+    "@storybook/addon-a11y",
+    "@storybook/addon-docs"
+  ],
+  "framework": "@storybook/sveltekit"
+};
+export default config;