feat(proxy): add full URL mode and refactor endpoint rewriting

- Add `isFullUrl` provider meta to treat base_url as complete API endpoint
- Remove hardcoded `?beta=true` from Claude adapter, pass through from client
- Refactor forwarder endpoint rewriting with proper query string handling
- Block provider switching when proxy is required but not running
- Add full URL toggle UI in endpoint field with i18n (zh/en/ja)

src/types.ts

@@ -151,6 +151,8 @@ export interface ProviderMeta {
   apiFormat?: "anthropic" | "openai_chat" | "openai_responses";
   // Claude 认证字段名
   apiKeyField?: ClaudeApiKeyField;
+  // 是否将 base_url 视为完整 API 端点（代理直接使用此 URL，不拼接路径）
+  isFullUrl?: boolean;
   // Prompt cache key for OpenAI-compatible endpoints (improves cache hit rate)
   promptCacheKey?: string;
 }