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/wifi/wifi_manager.h

@@ -49,3 +49,13 @@ EventGroupHandle_t wifi_manager_get_event_group(void);
  * Scan and print nearby APs.
  */
 void wifi_manager_scan_and_print(void);
+
+/**
+ * Check if WiFi credentials exist (NVS or build-time secrets).
+ */
+bool wifi_manager_has_credentials(void);
+
+/**
+ * Stop WiFi (for mode switching during onboarding).
+ */
+esp_err_t wifi_manager_stop(void);