feat: add WiFi onboarding captive portal

When no WiFi credentials are configured or connection fails, the device
automatically starts a soft AP (MimiClaw-XXXX) with a captive portal.
Users can configure WiFi, LLM, Telegram, Feishu, Proxy and Search
settings via a mobile-friendly web page, then the device restarts and
connects with the new credentials.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main/onboard/wifi_onboard.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include "esp_err.h"
+
+/**
+ * Start WiFi onboarding captive portal.
+ * Opens a soft AP, DNS hijacker, and HTTP configuration server.
+ * Blocks until the user submits credentials, then saves to NVS and restarts.
+ */
+esp_err_t wifi_onboard_start(void);