Fix potential division by zero in cache percentage calculation

Add guard to ensure denominator is non-zero before calculating the
cache percentage. This prevents NaN when both input_tokens and
cache_read_input_tokens are 0.

Author: ejc3

web/app/routes/_index.tsx

@@ -711,7 +711,8 @@ export default function Index() {
                                 <span className="font-mono text-gray-600">
                                   <span className="font-medium text-gray-900">{(request.response.body.usage.output_tokens || 0).toLocaleString()}</span> out
                                 </span>
-                                {request.response.body.usage.cache_read_input_tokens && (
+                                {request.response.body.usage.cache_read_input_tokens &&
+                                  ((request.response.body.usage.input_tokens || 0) + (request.response.body.usage.cache_read_input_tokens || 0)) > 0 && (
                                   <span className="font-mono bg-green-50 text-green-700 px-1.5 py-0.5 rounded">
                                     {Math.round((request.response.body.usage.cache_read_input_tokens / ((request.response.body.usage.input_tokens || 0) + (request.response.body.usage.cache_read_input_tokens || 0))) * 100)}% cached
                                   </span>