Fix web UI: captive portal DNS + chunked HTML delivery

Phone browsers do a captive portal check (DNS + HTTP) when joining a
new WiFi AP. Without a DNS server the DNS query hangs forever and the
phone blocks all HTTP traffic to the network — page never loads.

Added DNSServer resolving all queries to 192.168.4.1. handleNotFound
now 302-redirects to / so captive portal probes get the main page.

Replaced single 15 KB send_P() with chunked transfer encoding in 512B
pieces with yield() between each chunk, keeping the WiFi stack responsive.

Made-with: Cursor

src/main.cpp

@@ -10,6 +10,7 @@
 #include <RadioLib.h>
 #include <WiFi.h>
 #include <WebServer.h>
+#include <DNSServer.h>
 #include <ESPmDNS.h>
 #include <ArduinoOTA.h>
 #include "driver/gpio.h"
@@ -35,6 +36,7 @@ CC1101 radio1(&mod1);
 CC1101 radio2(&mod2);
 
 static WebServer server(WEB_PORT);
+static DNSServer dnsServer;
 static Preferences preferences;
 
 // CC1101 valid discrete power levels in dBm (RadioLib only accepts these exact values)
@@ -2240,7 +2242,18 @@ window.addEventListener('resize',capDrawWave);
 )HTML";
 
 static void handleRoot() {
-  server.send_P(200, "text/html; charset=utf-8", kHtml, sizeof(kHtml) - 1);
+  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("");
 }
 
 static void handleLog() {
@@ -2531,9 +2544,8 @@ static void handleCaptureWave() {
 }
 
 static void handleNotFound() {
-  const String uri = server.uri();
-  logLine("[HTTP] 404 " + uri);
-  server.send(404, "text/plain", "404: Not found");
+  server.sendHeader("Location", "http://192.168.4.1/");
+  server.send(302, "text/plain", "Redirect");
 }
 
 static void handleSelfTest() {
@@ -2706,6 +2718,9 @@ void setup() {
   server.onNotFound(handleNotFound);
   server.begin();
 
+  dnsServer.start(53, "*", WiFi.softAPIP());
+  logLine("[DNS] Captive portal DNS on *:53 → " + WiFi.softAPIP().toString());
+
   // OTA firmware updates over WiFi (connect to 'killer' AP, upload via PlatformIO OTA)
   ArduinoOTA.setHostname("killer");
   ArduinoOTA.setPassword("killerpw");
@@ -2731,6 +2746,7 @@ void setup() {
 }
 
 void loop() {
+  dnsServer.processNextRequest();
   espNowTick();
   ArduinoOTA.handle();
   server.handleClient();