feat(providers): add APIFree support

Add APIFree as a built-in OpenAI-compatible provider. APIFree offers
agent-optimised models such as skywork-ai/skyclaw-v1 through an
OpenAI-compatible API at https://api.apifree.ai/agent/v1.

Changes:
- Register apifree provider in the provider registry
- Add config schema field
- Add documentation with configuration example
- Add provider tests, websocket channel tests, and webui tests
- Add provider icon in settings UI

tests/channels/test_websocket_channel.py

@@ -1034,6 +1034,8 @@ async def test_settings_api_returns_safe_subset_and_updates_whitelist(
         assert providers["skywork"]["default_api_base"] == "https://api.apifree.ai/v1"
         assert providers["ant_ling"]["label"] == "Ant Ling"
         assert providers["ant_ling"]["default_api_base"] == "https://api.ant-ling.com/v1"
+        assert providers["apifree"]["label"] == "APIFree"
+        assert providers["apifree"]["default_api_base"] == "https://api.apifree.ai/agent/v1"
         assert providers["atomic_chat"]["configured"] is False
         assert providers["atomic_chat"]["api_key_required"] is False
         assert providers["atomic_chat"]["default_api_base"] == "http://localhost:1337/v1"