feat(ui) : add dark mode toggle and remove inline dark: classes

- Add dark mode toggle button in top nav
- Add darkMode store with localStorage persistence
- Enable Tailwind class-based dark mode
- Import dark.css global overrides
- Remove inline dark: Tailwind classes (handled by global CSS)

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

frontend/nuxt.config.ts

@@ -2,6 +2,7 @@ export default defineNuxtConfig({
     compatibilityDate: '2025-07-15',
     devtools: {enabled: false},
     ssr: false,
+    css: ['~/assets/css/dark.css'],
     app: {
         baseURL: process.env.NODE_ENV === 'production'
             ? (process.env.NUXT_PUBLIC_APP_BASE || '/')