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/types.ts

@@ -144,6 +144,11 @@ export interface ProviderMeta {
 // Skill 同步方式
 export type SkillSyncMethod = "auto" | "symlink" | "copy";
 
+// Claude API 格式类型
+// - "anthropic": 原生 Anthropic Messages API 格式，直接透传
+// - "openai_chat": OpenAI Chat Completions 格式，需要格式转换
+export type ClaudeApiFormat = "anthropic" | "openai_chat";
+
 // 主页面显示的应用配置
 export interface VisibleApps {
   claude: boolean;