fix(provisioning): rip out racy DHCP-option dance, add /log endpoint

The previous "force DHCP DNS offer" code (esp_netif_dhcps_stop / set
DNS / set option / dhcps_start) ran AFTER softAP was already serving
beacons. A fast iOS join — and CNA probes follow DHCP within 1-2s —
could land in the middle of the dance and either get a stale lease
or be racing the server start. ESP-IDF's softAP DHCP server already
advertises the AP IP as DNS by default, so the dance was at best
redundant. Strip it. Also drop the WiFi.disconnect(true) call before
mode-switching to AP — there's nothing to disconnect from on a cold
boot, and disconnect(true) cycles the radio for no benefit.

Add /log: in-memory ring buffer (32 entries, FIFO) of HTTP requests
and AP-state events served as plain text at http://192.168.4.1/log.
Lets the user diagnose without USB serial — join the AP, browse to
the URL, see exactly which CNA paths iOS hit (or didn't).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main.cpp

@@ -8,7 +8,6 @@
 #include <Preferences.h>
 #include <LittleFS.h>
 #include <qrcode.h>
-#include "esp_netif.h"
 #include "config.h"
 #include "epd.h"
 #include "operation.h"
@@ -98,23 +97,30 @@ min-height:100vh;margin:0;background:#fdf6ee;color:#3a2e22;text-align:center}
 
 // ── Web server handlers ───────────────────────────────────────────────────────
 
-// Diagnostic — every probe we receive during provisioning gets logged
+// In-memory ring buffer of provisioning events (HTTP requests + WiFi
-// with method / URI / Host / User-Agent so a serial-monitor repro tells
+// events) so the user can browse them at http://192.168.4.1/log
-// us whether the client (iOS in particular) is actually hitting us, and
+// without needing serial. 32 entries is plenty for one provisioning
-// which CNA path it's using. Cheap; only runs in AP mode.
+// session and stays well under the ESP32's heap budget.
+static constexpr size_t LOG_CAP = 32;
+static String g_log[LOG_CAP];
+static size_t g_log_head = 0;
+static size_t g_log_count = 0;
+
+static void log_event(const String& line) {
+    Serial.println(line);
+    g_log[g_log_head] = String(millis()) + "ms " + line;
+    g_log_head = (g_log_head + 1) % LOG_CAP;
+    if (g_log_count < LOG_CAP) g_log_count++;
+}
+
 static void log_provisioning_request() {
     String method = server.method() == HTTP_GET   ? "GET"
                   : server.method() == HTTP_POST  ? "POST"
                   : server.method() == HTTP_HEAD  ? "HEAD"
                   : "OTHER";
-    Serial.print("[ap-http] ");
+    log_event("[http] " + method + " " + server.uri() +
-    Serial.print(method);
+              " host=" + server.hostHeader() +
-    Serial.print(" ");
+              " ua=" + server.header("User-Agent"));
-    Serial.print(server.uri());
-    Serial.print(" host=");
-    Serial.print(server.hostHeader());
-    Serial.print(" ua=");
-    Serial.println(server.header("User-Agent"));
 }
 
 static void handle_root() {
@@ -139,19 +145,33 @@ static void handle_connect() {
     server.send_P(200, "text/html", CONNECTING_HTML);
 }
 
+// Diagnostic page — plain text dump of the ring buffer, oldest first.
+// User browses to http://192.168.4.1/log after joining the AP. Tells
+// us whether iOS hit us at all and which CNA paths it tried.
+static void handle_log() {
+    String body = "pictureFrame provisioning log\n";
+    body += "uptime=" + String(millis()) + "ms entries=" + String(g_log_count) + "/" + String(LOG_CAP) + "\n\n";
+    size_t start = (g_log_count < LOG_CAP) ? 0 : g_log_head;
+    for (size_t i = 0; i < g_log_count; i++) {
+        body += g_log[(start + i) % LOG_CAP] + "\n";
+    }
+    server.sendHeader("Cache-Control", "no-store");
+    server.send(200, "text/plain", body);
+}
+
 static void on_ap_event(WiFiEvent_t event, WiFiEventInfo_t info) {
     switch (event) {
         case ARDUINO_EVENT_WIFI_AP_START:
-            Serial.println("[ap-event] AP started");
+            log_event("[wifi] AP started");
             break;
         case ARDUINO_EVENT_WIFI_AP_STACONNECTED:
-            Serial.println("[ap-event] station associated");
+            log_event("[wifi] station associated");
             break;
         case ARDUINO_EVENT_WIFI_AP_STAIPASSIGNED:
-            Serial.println("[ap-event] DHCP lease handed out");
+            log_event("[wifi] DHCP lease handed out");
             break;
         case ARDUINO_EVENT_WIFI_AP_STADISCONNECTED:
-            Serial.println("[ap-event] station disconnected");
+            log_event("[wifi] station disconnected");
             break;
         default:
             break;
@@ -185,6 +205,7 @@ static void enter_provisioning(const String& mac, bool retry = false) {
     // canonical signal that this is a captive network.
     server.on("/",                          HTTP_GET,  handle_root);
     server.on("/connect",                   HTTP_POST, handle_connect);
+    server.on("/log",                       HTTP_GET,  handle_log);
     server.on("/hotspot-detect.html",       handle_root);  // iOS / macOS
     server.on("/library/test/success.html", handle_root);  // iOS legacy
     server.on("/generate_204",              handle_root);  // Android
@@ -198,7 +219,6 @@ static void enter_provisioning(const String& mac, bool retry = false) {
     server.collectHeaders(tracked_headers, 2);
 
     WiFi.onEvent(on_ap_event);
-    WiFi.disconnect(true);
     WiFi.mode(WIFI_AP);
     WiFi.softAP(apSsid.c_str());
 
@@ -207,31 +227,11 @@ static void enter_provisioning(const String& mac, bool retry = false) {
     // so the captive-detect window is deterministic.
     WiFi.setSleep(false);
 
-    // Belt-and-braces DHCP DNS advertisement: ESP-IDF's softAP DHCP server
+    // Trust the default ESP-IDF softAP DHCP server: it offers the AP IP
-    // *should* offer the AP IP as DNS by default, but if the client comes
+    // as DNS automatically. We previously did a stop / set-options /
-    // in with cached cellular DNS (8.8.8.8 etc) the captive DNS hijack
+    // restart dance on top of softAP to "force" the DNS offer — but
-    // gets bypassed entirely and iOS resolves captive.apple.com to the
+    // that races with iOS's DHCP request (a fast join can hit DHCP-stop
-    // real internet. Stopping the DHCP server, setting DNS-info, flipping
+    // mid-handshake) and likely caused more failures than it fixed.
-    // the offer-DNS flag, then restarting forces every lease to carry our
-    // address as the primary resolver.
-    esp_netif_t *ap_netif = esp_netif_get_handle_from_ifkey("WIFI_AP_DEF");
-    if (ap_netif) {
-        esp_netif_dhcps_stop(ap_netif);
-
-        esp_netif_ip_info_t ip;
-        esp_netif_get_ip_info(ap_netif, &ip);
-        esp_netif_dns_info_t dns_info = {};
-        dns_info.ip.u_addr.ip4.addr = ip.ip.addr;
-        dns_info.ip.type = ESP_IPADDR_TYPE_V4;
-        esp_netif_set_dns_info(ap_netif, ESP_NETIF_DNS_MAIN, &dns_info);
-
-        uint8_t offer_dns = 1;
-        esp_netif_dhcps_option(ap_netif, ESP_NETIF_OP_SET,
-                               ESP_NETIF_DOMAIN_NAME_SERVER,
-                               &offer_dns, sizeof(offer_dns));
-
-        esp_netif_dhcps_start(ap_netif);
-    }
 
     server.begin();
     dns.setErrorReplyCode(DNSReplyCode::NoError);