refactor: redesign planos interface with design system patterns

- Create CreatePlanModal component with Headless UI Dialog
- Implement dark mode support throughout plans UI
- Update plans/page.tsx with professional card layout
- Update plans/[id]/page.tsx with consistent styling
- Add proper spacing, typography, and color consistency
- Implement smooth animations and transitions
- Add success/error message feedback
- Improve form UX with better input styling

front-end-agency/app/globals.css

@@ -47,7 +47,7 @@ html.dark {
 
 @layer base {
   * {
-    font-family: var(--font-inter), ui-sans-serif, system-ui, sans-serif;
+    font-family: var(--font-arimo), ui-sans-serif, system-ui, sans-serif;
   }
 
   a,