Footer says what a latency figure is (closes #208)

The old line said every check is a GET and that CORS may affect
figures; the S3 targets use HEAD and a no-cors check counts any answer.

Model: opus-5-5

web/src/main.js

@@ -924,7 +924,7 @@ export function buildUI(state) {
       </div>
 
       <footer class="mt-8 text-center text-gray-600 text-xs">
-        <p>Latency measured via GET requests | CORS restrictions may affect some measurements</p>
+        <p>Latency: the time until a target's response headers arrive</p>
         <p class="mt-2">
           <span class="inline-block w-3 h-3 rounded-full bg-green-500 mr-1 align-middle"></span>&lt;50ms
           <span class="inline-block w-3 h-3 rounded-full bg-lime-500 mr-1 ml-3 align-middle"></span>&lt;100ms