feat(provider): 添加独立的 Provider 模块管理模型提供商

实现可扩展的 Provider 系统，支持动态注册自定义提供商：

Core 模块 (packages/core/src/provider/):
- types.ts: Provider 相关类型定义
- builtin/: 内置提供商 (Anthropic, OpenAI, DeepSeek)
- registry.ts: ProviderRegistry 单例类
- config.ts: 配置持久化 (~/.ai-terminal-assistant/providers.json)
- utils.ts: 连接测试等工具函数

Server API (packages/server/src/routes/providers.ts):
- GET/POST/PUT/DELETE /providers 提供商管理
- POST /providers/:id/test 连接测试
- 自定义模型管理接口

Frontend (packages/ui/):
- ProvidersPanel 组件用于管理提供商
- API client 函数和类型定义

主要功能：
- 支持动态注册 OpenAI 兼容服务 (Ollama, vLLM 等)
- 每个提供商独立的 API Key 配置
- 预设模型列表 + 自定义模型输入
- 连接测试验证

packages/core/src/provider/builtin/anthropic.ts

@@ -0,0 +1,50 @@
+/**
+ * Anthropic Provider Definition
+ */
+
+import { createAnthropic } from '@ai-sdk/anthropic';
+import type { ProviderInfo, ProviderFactory } from '../types.js';
+
+export const anthropicProvider: ProviderInfo = {
+  id: 'anthropic',
+  name: 'Anthropic',
+  description: 'Claude AI models by Anthropic',
+  builtin: true,
+  apiKeyEnvVar: 'ANTHROPIC_API_KEY',
+  models: [
+    {
+      id: 'claude-sonnet-4-20250514',
+      name: 'Claude Sonnet 4',
+      capabilities: { vision: true, functionCalling: true, streaming: true },
+      contextWindow: 200000,
+      maxOutput: 8192,
+    },
+    {
+      id: 'claude-3-5-sonnet-20241022',
+      name: 'Claude 3.5 Sonnet',
+      capabilities: { vision: true, functionCalling: true, streaming: true },
+      contextWindow: 200000,
+      maxOutput: 8192,
+    },
+    {
+      id: 'claude-3-opus-20240229',
+      name: 'Claude 3 Opus',
+      capabilities: { vision: true, functionCalling: true, streaming: true },
+      contextWindow: 200000,
+      maxOutput: 4096,
+    },
+    {
+      id: 'claude-3-haiku-20240307',
+      name: 'Claude 3 Haiku',
+      capabilities: { vision: true, functionCalling: true, streaming: true },
+      contextWindow: 200000,
+      maxOutput: 4096,
+    },
+  ],
+  allowCustomModels: true,
+};
+
+export const anthropicFactory: ProviderFactory = ({ apiKey, baseUrl }) => {
+  const client = createAnthropic({ apiKey, baseURL: baseUrl });
+  return (model) => client(model);
+};