feat(02-01): design system, color-mode, sitemap config

- Brand color #85cb85 as CSS @theme with full shade palette
- app.config.ts maps Nuxt UI primary to brand
- colorMode with cookie storage, dark default, no FOUC
- i18n baseUrl and site.url for absolute SEO URLs
- Static og:image placeholder in public/

app.config.ts

@@ -0,0 +1,7 @@
+export default defineAppConfig({
+  ui: {
+    colors: {
+      primary: 'brand',
+    },
+  },
+})