feat(claude): add API format selector for third-party providers

Replace the OpenRouter-specific compatibility toggle with a generic
API format selector that allows all Claude providers to choose between:

- Anthropic Messages (native): Direct passthrough, no conversion
- OpenAI Chat Completions: Enables Anthropic ↔ OpenAI format conversion

Changes:
- Add ClaudeApiFormat type ("anthropic" | "openai_chat") to types.ts
- Replace openRouterCompatToggle with apiFormat dropdown in ClaudeFormFields
- Update ProviderForm to manage apiFormat state via settingsConfig.api_format
- Refactor claude.rs: add get_api_format() method, update needs_transform()
- Maintain backward compatibility with legacy openrouter_compat_mode field
- Update i18n translations (zh, en, ja)

src/i18n/locales/en.json

@@ -472,8 +472,10 @@
     "anthropicModel": "Main Model",
     "anthropicSmallFastModel": "Fast Model",
     "anthropicReasoningModel": "Reasoning Model (Thinking)",
-    "openrouterCompatMode": "OpenRouter Compatibility Mode",
-    "openrouterCompatModeHint": "Use OpenAI Chat Completions interface and convert to Anthropic SSE.",
+    "apiFormat": "API Format",
+    "apiFormatHint": "Select the input format for the provider's API",
+    "apiFormatAnthropic": "Anthropic Messages (Native)",
+    "apiFormatOpenAIChat": "OpenAI Chat Completions (Requires conversion)",
     "anthropicDefaultHaikuModel": "Default Haiku Model",
     "anthropicDefaultSonnetModel": "Default Sonnet Model",
     "anthropicDefaultOpusModel": "Default Opus Model",

src/i18n/locales/ja.json

@@ -472,8 +472,10 @@
     "anthropicModel": "メインモデル",
     "anthropicSmallFastModel": "高速モデル",
     "anthropicReasoningModel": "推論モデル（Thinking）",
-    "openrouterCompatMode": "OpenRouter 互換モード",
-    "openrouterCompatModeHint": "OpenAI Chat Completions インターフェースを使用し、Anthropic SSE に変換します。",
+    "apiFormat": "API フォーマット",
+    "apiFormatHint": "プロバイダー API の入力フォーマットを選択",
+    "apiFormatAnthropic": "Anthropic Messages（ネイティブ）",
+    "apiFormatOpenAIChat": "OpenAI Chat Completions（変換が必要）",
     "anthropicDefaultHaikuModel": "既定 Haiku モデル",
     "anthropicDefaultSonnetModel": "既定 Sonnet モデル",
     "anthropicDefaultOpusModel": "既定 Opus モデル",

src/i18n/locales/zh.json

@@ -472,8 +472,10 @@
     "anthropicModel": "主模型",
     "anthropicSmallFastModel": "快速模型",
     "anthropicReasoningModel": "推理模型 (Thinking)",
-    "openrouterCompatMode": "OpenRouter 兼容模式",
-    "openrouterCompatModeHint": "使用 OpenAI Chat Completions 接口并转换为 Anthropic SSE。",
+    "apiFormat": "API 格式",
+    "apiFormatHint": "选择供应商 API 的输入格式",
+    "apiFormatAnthropic": "Anthropic Messages (原生)",
+    "apiFormatOpenAIChat": "OpenAI Chat Completions (需转换)",
     "anthropicDefaultHaikuModel": "Haiku 默认模型",
     "anthropicDefaultSonnetModel": "Sonnet 默认模型",
     "anthropicDefaultOpusModel": "Opus 默认模型",