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/

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/assets/css/main.css

@@ -0,0 +1,16 @@
+@import "tailwindcss";
+@import "@nuxt/ui";
+
+@theme {
+  --color-brand-50: #f0faf0;
+  --color-brand-100: #dcf3dc;
+  --color-brand-200: #bbe8bb;
+  --color-brand-300: #8dd98d;
+  --color-brand-400: #a3d6a3;
+  --color-brand-500: #85cb85;
+  --color-brand-600: #5aaa5a;
+  --color-brand-700: #3f8c3f;
+  --color-brand-800: #2e6b2e;
+  --color-brand-900: #1f4f1f;
+  --color-brand-950: #122d12;
+}