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/provider.rs

@@ -172,29 +172,6 @@ pub struct ProviderTestConfig {
     pub max_retries: Option<u32>,
 }
 
-/// 供应商单独的代理配置
-#[derive(Debug, Clone, Serialize, Deserialize, Default)]
-pub struct ProviderProxyConfig {
-    /// 是否启用单独配置（false 时使用全局/系统代理）
-    #[serde(default)]
-    pub enabled: bool,
-    /// 代理类型：http, https, socks5
-    #[serde(rename = "proxyType", skip_serializing_if = "Option::is_none")]
-    pub proxy_type: Option<String>,
-    /// 代理主机
-    #[serde(rename = "proxyHost", skip_serializing_if = "Option::is_none")]
-    pub proxy_host: Option<String>,
-    /// 代理端口
-    #[serde(rename = "proxyPort", skip_serializing_if = "Option::is_none")]
-    pub proxy_port: Option<u16>,
-    /// 代理用户名（可选）
-    #[serde(rename = "proxyUsername", skip_serializing_if = "Option::is_none")]
-    pub proxy_username: Option<String>,
-    /// 代理密码（可选）
-    #[serde(rename = "proxyPassword", skip_serializing_if = "Option::is_none")]
-    pub proxy_password: Option<String>,
-}
-
 /// 认证绑定来源
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, Default)]
 #[serde(rename_all = "snake_case")]
@@ -262,9 +239,6 @@ pub struct ProviderMeta {
     /// 供应商单独的模型测试配置
     #[serde(rename = "testConfig", skip_serializing_if = "Option::is_none")]
     pub test_config: Option<ProviderTestConfig>,
-    /// 供应商单独的代理配置
-    #[serde(rename = "proxyConfig", skip_serializing_if = "Option::is_none")]
-    pub proxy_config: Option<ProviderProxyConfig>,
     /// Claude API 格式（仅 Claude 供应商使用）
     /// - "anthropic": 原生 Anthropic Messages API，直接透传
     /// - "openai_chat": OpenAI Chat Completions 格式，需要转换