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/CMakeLists.txt

@@ -21,6 +21,7 @@ idf_component_register(
         "tools/tool_get_time.c"
         "tools/tool_files.c"
         "skills/skill_loader.c"
+        "onboard/wifi_onboard.c"
     INCLUDE_DIRS
         "."
     REQUIRES