feat: add SiliconFlow.cn chat completion and embedding support (#5316)

* feat: add SiliconFlow.cn endpoint support and embedding vectors

Chat completion:
- Add endpoint selection dropdown (Global/.com vs China/.cn) to existing
  SiliconFlow provider, following the Z.AI endpoint pattern
- Backend switches API URL based on selected endpoint
- Add /api-url slash command support for endpoint switching

Embeddings:
- Add SiliconFlow as a vector/embedding source (OpenAI-compatible)
- Support both .com and .cn endpoints via siliconflow_endpoint setting
  borrowed from the main connection panel (Vertex AI pattern)
- Superset model list with platform attribution (.cn) markers
- Models: Qwen3-Embedding (0.6B/4B/8B) + BGE/BCE models (.cn only)

* Add filter by models type

* Load embedding models from endpoint

* Improve api-url command declaration

* Support endpoint override in custom-request service

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Co-authored-by: Cohee <18619528+Cohee1207@users.noreply.github.com>

public/index.html

@@ -3556,6 +3556,11 @@
                             <div data-for="api_key_siliconflow" class="neutral_warning" data-i18n="For privacy reasons, your API key will be hidden after you click 'Connect'.">
                                 For privacy reasons, your API key will be hidden after you click 'Connect'.
                             </div>
+                            <h4 data-i18n="SiliconFlow Endpoint">SiliconFlow Endpoint</h4>
+                            <select id="siliconflow_endpoint">
+                                <option value="global" data-i18n="Global (siliconflow.com)">Global (siliconflow.com)</option>
+                                <option value="cn" data-i18n="China (siliconflow.cn)">China (siliconflow.cn)</option>
+                            </select>
                             <h4>SiliconFlow Model</h4>
                             <select id="model_siliconflow_select">
                                 <option value="" data-i18n="-- Connect to the API --">-- Connect to the API --</option>