refactor: remove per-provider proxy config feature

The per-provider proxy configuration (meta.proxyConfig) is removed
because its scope is too narrow and covered by global proxy settings
and proxy takeover mode. Users can achieve the same result via the
global proxy panel.

Changes:
- Remove ProviderProxyConfig type (frontend TS + backend Rust)
- Remove ProviderAdvancedConfig proxy UI block, keep testConfig/pricingConfig
- Simplify http_client: delete build_proxy_url_from_config,
  build_client_for_provider, get_for_provider
- Simplify forwarder/stream_check/model_fetch to use global client
- Remove i18n keys (en/zh/ja)
- Fix pre-existing test bug in transform.rs (extra None arg)

src/types.ts

@@ -109,22 +109,6 @@ export interface ProviderTestConfig {
   maxRetries?: number;
 }
 
-// 供应商单独的代理配置
-export interface ProviderProxyConfig {
-  // 是否启用单独配置（false 时使用全局/系统代理）
-  enabled: boolean;
-  // 代理类型：http, https, socks5
-  proxyType?: "http" | "https" | "socks5";
-  // 代理主机
-  proxyHost?: string;
-  // 代理端口
-  proxyPort?: number;
-  // 代理用户名（可选）
-  proxyUsername?: string;
-  // 代理密码（可选）
-  proxyPassword?: string;
-}
-
 export type AuthBindingSource = "provider_config" | "managed_account";
 
 export interface AuthBinding {
@@ -149,8 +133,6 @@ export interface ProviderMeta {
   partnerPromotionKey?: string;
   // 供应商单独的模型测试配置
   testConfig?: ProviderTestConfig;
-  // 供应商单独的代理配置
-  proxyConfig?: ProviderProxyConfig;
   // 供应商成本倍率
   costMultiplier?: string;
   // 供应商计费模式来源