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

docs/configuration.md

@@ -154,6 +154,7 @@ ANTHROPIC_API_KEY="$(bw get password api/anthropic)" nanobot agent
 | `mimo` | LLM (MiMo) | [platform.xiaomimimo.com](https://platform.xiaomimimo.com) |
 | `longcat` | LLM (LongCat) | [longcat.chat](https://longcat.chat/platform/docs/zh/) |
 | `ant_ling` | LLM (Ant Ling / 蚂蚁百灵) | [developer.ant-ling.com](https://developer.ant-ling.com/en/docs/api-reference/openai/) |
+| `apifree` | LLM (APIFree) | [apifree.ai](https://www.apifree.ai) |
 | `ollama` | LLM (local, Ollama) | — |
 | `lm_studio` | LLM (local, LM Studio) | — |
 | `atomic_chat` | LLM (local, [Atomic Chat](https://atomic.chat/)) | — |
@@ -504,6 +505,33 @@ Official OpenAI-compatible model names include `Ling-2.6-1T`,
 
 </details>
 
+<details>
+<summary><b>APIFree (OpenAI-compatible)</b></summary>
+
+APIFree is available through nanobot's built-in OpenAI-compatible provider flow.
+The default API base points to `https://api.apifree.ai/agent/v1`, so you usually
+only need to set `apiKey`.
+
+```json
+{
+  "providers": {
+    "apifree": {
+      "apiKey": "${APIFREE_API_KEY}"
+    }
+  },
+  "agents": {
+    "defaults": {
+      "provider": "apifree",
+      "model": "skywork-ai/skyclaw-v1"
+    }
+  }
+}
+```
+
+Available models include `skywork-ai/skyclaw-v1`.
+
+</details>
+
 <details>
 <summary><b>Custom Provider (Any OpenAI-compatible API)</b></summary>