feat: dark/light theming system + design tokens (OMC design system port)

- Restructure design tokens into :root/.dark raw vars mapped via @theme inline
- Add light/dark/system theming: useTheme hook + FOUC guard + ThemeToggle
- Keep iamcavalli palette (primary #1A463C, accent #DEF168) and Geist fonts
- Derive brand-consistent dark palette (verde-nero bg, lightened primary)
- Add scrollbar utilities (.no-scrollbar/.thin-scrollbar)
- Install tailwindcss-animate, register via @plugin (Tailwind v4)
- Mount ThemeToggle in admin sidebar footer

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -34,6 +34,7 @@
     "react-dom": "19.2.4",
     "react-hook-form": "^7.75.0",
     "tailwind-merge": "^3.6.0",
+    "tailwindcss-animate": "^1.0.7",
     "zod": "^4.4.3"
   },
   "devDependencies": {