Fix web UI: sendContent length-aware overload, no heap String alloc

Made-with: Cursor

src/main.cpp

@@ -2250,7 +2250,7 @@ static void handleRoot() {
   }
   server.setContentLength(sizeof(kHtml) - 1);
   server.send(200, "text/html; charset=utf-8", "");
-  server.sendContent(kHtml);
+  server.sendContent(kHtml, sizeof(kHtml) - 1);
 }
 
 static void handleLog() {