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-tauri/src/services/model_fetch.rs

@@ -41,7 +41,7 @@ pub async fn fetch_models(
     }
 
     let models_url = build_models_url(base_url, is_full_url)?;
-    let client = crate::proxy::http_client::get_for_provider(None);
+    let client = crate::proxy::http_client::get();
 
     let response = client
         .get(&models_url)

src-tauri/src/services/stream_check.rs

@@ -218,9 +218,8 @@ impl StreamCheckService {
             .or_else(|| adapter.extract_auth(provider))
             .ok_or_else(|| AppError::Message("API Key not found".to_string()))?;
 
-        // 获取 HTTP 客户端：优先使用供应商单独代理配置，否则使用全局客户端
-        let proxy_config = provider.meta.as_ref().and_then(|m| m.proxy_config.as_ref());
-        let client = crate::proxy::http_client::get_for_provider(proxy_config);
+        // 获取 HTTP 客户端
+        let client = crate::proxy::http_client::get();
         let request_timeout = std::time::Duration::from_secs(config.timeout_secs);
 
         let model_to_test = Self::resolve_test_model(app_type, provider, config);
@@ -659,9 +658,8 @@ impl StreamCheckService {
         config: &StreamCheckConfig,
         start: Instant,
     ) -> Result<StreamCheckResult, AppError> {
-        // 获取 HTTP 客户端：优先使用供应商单独代理配置，否则使用全局客户端
-        let proxy_config = provider.meta.as_ref().and_then(|m| m.proxy_config.as_ref());
-        let client = crate::proxy::http_client::get_for_provider(proxy_config);
+        // 获取 HTTP 客户端
+        let client = crate::proxy::http_client::get();
         let request_timeout = std::time::Duration::from_secs(config.timeout_secs);
 
         let model_to_test = Self::resolve_test_model(app_type, provider, config);