fix: 完善大模型供应商下拉

- 移除当前协议不兼容的 Perplexity 预设
- 限制供应商与模型列表高度并支持滚动

frontend/src/locales/en-US.json

@@ -8648,7 +8648,6 @@
     "providerOllama": "Ollama",
     "providerOpenai": "OpenAI",
     "providerOpenrouter": "OpenRouter",
-    "providerPerplexity": "Perplexity",
     "providerQwen": "Qwen",
     "providerSiliconflow": "SiliconFlow",
     "providerSpark": "iFlytek Spark",