Add warm neutral background to light theme

Set light theme background to warm cream (#F5F3EF) with white cards,
softer text (#2C2C2C), and a surface variant (#EDE9E3) to eliminate
the flat all-white appearance.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/plugins/vuetify.ts

@@ -33,6 +33,11 @@ export default createVuetify({
           warning: '#FFC107',
           info: '#2196F3',
           success: '#4CAF50',
+          background: '#F5F3EF',
+          surface: '#FFFFFF',
+          'surface-variant': '#EDE9E3',
+          'on-background': '#2C2C2C',
+          'on-surface': '#2C2C2C',
         }
       }
     }