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/i18n/locales/en.json

@@ -943,11 +943,6 @@
     "testPrompt": "Test Prompt",
     "degradedThreshold": "Degraded Threshold (ms)",
     "maxRetries": "Max Retries",
-    "proxyConfig": "Proxy Config",
-    "useCustomProxy": "Use separate proxy",
-    "proxyConfigDesc": "Configure separate network proxy for this provider. Uses system proxy or global settings when disabled.",
-    "proxyUsername": "Username (optional)",
-    "proxyPassword": "Password (optional)",
     "pricingConfig": "Pricing Config",
     "useCustomPricing": "Use separate config",
     "pricingConfigDesc": "Configure separate pricing parameters for this provider. Uses global defaults when disabled.",

src/i18n/locales/ja.json

@@ -943,11 +943,6 @@
     "testPrompt": "テストプロンプト",
     "degradedThreshold": "低下閾値（ミリ秒）",
     "maxRetries": "最大リトライ回数",
-    "proxyConfig": "プロキシ設定",
-    "useCustomProxy": "個別プロキシを使用",
-    "proxyConfigDesc": "このプロバイダーに個別のネットワークプロキシを設定します。無効の場合はシステムプロキシまたはグローバル設定を使用します。",
-    "proxyUsername": "ユーザー名（任意）",
-    "proxyPassword": "パスワード（任意）",
     "pricingConfig": "課金設定",
     "useCustomPricing": "個別設定を使用",
     "pricingConfigDesc": "このプロバイダーに個別の課金パラメータを設定します。無効の場合はグローバル設定を使用します。",

src/i18n/locales/zh.json

@@ -944,11 +944,6 @@
     "testPrompt": "测试提示词",
     "degradedThreshold": "降级阈值（毫秒）",
     "maxRetries": "最大重试次数",
-    "proxyConfig": "代理配置",
-    "useCustomProxy": "使用单独代理",
-    "proxyConfigDesc": "为此供应商配置单独的网络代理，不启用时使用系统代理或全局设置。",
-    "proxyUsername": "用户名（可选）",
-    "proxyPassword": "密码（可选）",
     "pricingConfig": "计费配置",
     "useCustomPricing": "使用单独配置",
     "pricingConfigDesc": "为此供应商配置单独的计费参数，不启用时使用全局默认配置。",