Fix web UI white screen: restore send_P with strict cache-busting headers

Made-with: Cursor

src/main.cpp

@@ -2240,18 +2240,14 @@ window.addEventListener('resize',capDrawWave);
 )HTML";
 
 static void handleRoot() {
-  server.setContentLength(CONTENT_LENGTH_UNKNOWN);
-  server.send(200, "text/html; charset=utf-8", "");
-  const size_t total = sizeof(kHtml) - 1;
-  size_t off = 0;
-  while (off < total) {
-    size_t n = total - off;
-    if (n > 512) n = 512;
-    server.sendContent_P(kHtml + off, n);
-    off += n;
-    yield();
-  }
-  server.sendContent("");
+  // Prevent browser caching so it always fetches the full page
+  server.sendHeader("Cache-Control", "no-cache, no-store, must-revalidate");
+  server.sendHeader("Pragma", "no-cache");
+  server.sendHeader("Expires", "0");
+  
+  // Send directly from flash. WebServer::send_P handles Content-Length and TCP chunking
+  // internally without allocating a 15 KB String on the heap.
+  server.send_P(200, "text/html; charset=utf-8", kHtml, sizeof(kHtml) - 1);
 }
 
 static void handleLog() {