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/tailwind.config.ts

@@ -1,6 +1,7 @@
 import type {Config} from 'tailwindcss'
 
 export default <Partial<Config>>{
+    darkMode: 'class',
     theme: {
         extend: {
             fontFamily: {