Add fade-in animation for model fallback toast notification

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

frontend/index.html

@@ -45,6 +45,7 @@
             },
             animation: {
               'slow-ping': 'ping 2s cubic-bezier(0, 0, 0.2, 1) infinite',
+              'fade-in': 'fadeIn 0.3s ease-out',
             },
             keyframes: {
               ping: {
@@ -53,6 +54,10 @@
                   opacity: '0'
                 }
               },
+              fadeIn: {
+                '0%': { opacity: '0', transform: 'translateY(8px)' },
+                '100%': { opacity: '1', transform: 'translateY(0)' },
+              },
             }
           },
         },