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/components/providers/EditProviderDialog.tsx

@@ -154,7 +154,7 @@ export function EditProviderDialog({
   }, [
     open, // 修复：编辑保存后再次打开显示旧数据，依赖 open 确保每次打开时重新读取最新 provider 数据
     provider?.id, // 只依赖 ID，provider 对象更新不会触发重新计算
-    provider?.meta, // 需要依赖 meta 以便正确初始化 testConfig 和 proxyConfig
+    provider?.meta, // 需要依赖 meta 以便正确初始化 testConfig
     initialSettingsConfig,
   ]);