feat(proxy): flag managed-OAuth providers as routing-required

Managed-OAuth providers (github_copilot / codex_oauth / xai_oauth) need
proxy takeover to inject credentials, but the "needs routing" badge and
the switch-time warning only keyed off apiFormat — missing OAuth providers
whose upstream format is native (e.g. the new Codex xAI Grok OAuth preset
on openai_responses).

- Add isOAuthProviderType / OAUTH_PROVIDER_TYPES as the SSOT for OAuth types
- Extract providerNeedsRouting() as the authoritative predicate: managed
  OAuth always needs routing (the backend rejects these accounts' direct
  connection) regardless of apiFormat or Claude Desktop mode
- ProviderCard badges (claude / codex / claude-desktop) and the switch
  warning consume the shared predicate instead of raw apiFormat
- Gate the warning on per-app routing readiness: claude-desktop uses
  isProxyRunning (backend takeover status has no such field), other apps
  use isProxyTakeover — fixes false warnings on Desktop and missing
  warnings when the proxy runs but the app isn't taken over
- Force proxy mode for all managed-OAuth providers in the Claude Desktop
  form (lock the mode toggle), not only xai_oauth
- Add unit tests for providerNeedsRouting, the switch routing gate, and
  Desktop OAuth preset enforcement
- i18n(zh/en/ja/zh-TW): add notifications.proxyReasonManagedOAuth

src/i18n/locales/en.json

@@ -259,6 +259,8 @@
     "proxyReasonOpenAIResponses": "uses OpenAI Responses API format",
     "proxyReasonAnthropicMessages": "uses Anthropic Messages API format",
     "proxyReasonFullUrl": "has full URL connection mode enabled",
+    "proxyReasonManagedOAuth": "uses managed OAuth login (token injected by local routing)",
+    "proxyReasonRoutingRequired": "requires local routing to process requests",
     "openAIFormatHint": "This provider uses OpenAI-compatible format and requires the routing service to be enabled",
     "copilotProxyHint": "GitHub Copilot as a Claude provider always requires local routing; routing automatically selects Chat Completions or Responses based on the current model.",
     "openLinkFailed": "Failed to open link",

src/i18n/locales/ja.json

@@ -259,6 +259,8 @@
     "proxyReasonOpenAIResponses": "OpenAI Responses API フォーマットを使用しており",
     "proxyReasonAnthropicMessages": "Anthropic Messages API フォーマットを使用しており",
     "proxyReasonFullUrl": "完全 URL 接続モードが有効になっており",
+    "proxyReasonManagedOAuth": "マネージド OAuth ログイン（トークンはローカルルーティングが注入）を使用しており",
+    "proxyReasonRoutingRequired": "リクエスト処理にローカルルーティングが必要であり",
     "openAIFormatHint": "このプロバイダーは OpenAI 互換フォーマットを使用しており、ルーティングサービスの有効化が必要です",
     "copilotProxyHint": "GitHub Copilot を Claude プロバイダーとして使用する場合、ローカルルーティングが常に必要です。ルーティングは現在のモデルに応じて Chat Completions または Responses を自動的に選択します。",
     "openLinkFailed": "リンクを開けませんでした",

src/i18n/locales/zh-TW.json

@@ -259,6 +259,8 @@
     "proxyReasonOpenAIResponses": "使用 OpenAI Responses API 格式",
     "proxyReasonAnthropicMessages": "使用 Anthropic Messages API 格式",
     "proxyReasonFullUrl": "開啟了完整 URL 連線模式",
+    "proxyReasonManagedOAuth": "使用託管 OAuth 登入（權杖由本機路由注入）",
+    "proxyReasonRoutingRequired": "需要本機路由處理請求",
     "openAIFormatHint": "此供應商使用 OpenAI 相容格式，需要開啟路由服務才能正常使用",
     "copilotProxyHint": "GitHub Copilot 作為 Claude 供應商時始終需要本地路由；路由會根據目前模型自動選擇 Chat Completions 或 Responses。",
     "openLinkFailed": "連結開啟失敗",

src/i18n/locales/zh.json

@@ -259,6 +259,8 @@
     "proxyReasonOpenAIResponses": "使用 OpenAI Responses 接口格式",
     "proxyReasonAnthropicMessages": "使用 Anthropic Messages 接口格式",
     "proxyReasonFullUrl": "开启了完整 URL 连接模式",
+    "proxyReasonManagedOAuth": "使用托管 OAuth 登录（令牌由本地路由注入）",
+    "proxyReasonRoutingRequired": "需要本地路由处理请求",
     "openAIFormatHint": "此供应商使用 OpenAI 兼容格式，需要开启路由服务才能正常使用",
     "copilotProxyHint": "GitHub Copilot 作为 Claude 供应商时始终需要本地路由；路由会根据当前模型自动选择 Chat Completions 或 Responses。",
     "openLinkFailed": "链接打开失败",