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/proxy/forwarder.rs

@@ -1347,12 +1347,8 @@ impl RequestForwarder {
             self.non_streaming_timeout
         };
 
-        // 解析上游代理 URL（供应商单独代理 > 全局代理 > 无）
-        let proxy_config = provider.meta.as_ref().and_then(|m| m.proxy_config.as_ref());
-        let upstream_proxy_url: Option<String> = proxy_config
-            .filter(|c| c.enabled)
-            .and_then(super::http_client::build_proxy_url_from_config)
-            .or_else(super::http_client::get_current_proxy_url);
+        // 获取全局代理 URL
+        let upstream_proxy_url: Option<String> = super::http_client::get_current_proxy_url();
 
         // SOCKS5 代理不支持 CONNECT 隧道，需要用 reqwest
         let is_socks_proxy = upstream_proxy_url
@@ -1368,7 +1364,7 @@ impl RequestForwarder {
         let response = if is_socks_proxy {
             // SOCKS5 代理：只能走 reqwest（不支持 header case 保留）
             log::debug!("[Forwarder] Using reqwest for SOCKS5 proxy");
-            let client = super::http_client::get_for_provider(proxy_config);
+            let client = super::http_client::get();
             let mut request = client.post(&url);
             if !self.non_streaming_timeout.is_zero() {
                 request = request.timeout(self.non_streaming_timeout);