feat(proxy): add isFullUrl toggle for full API endpoint mode

- provider.rs/types.ts: add is_full_url field to ProviderMeta
- forwarder.rs: when isFullUrl is set, use base_url directly instead of
  appending endpoint path; also handle query passthrough and strip
  beta=true when transforming to /v1/chat/completions
- EndpointField.tsx: add Link2 icon toggle button for full URL mode
- ClaudeFormFields.tsx: pass through isFullUrl/onFullUrlChange props
- ProviderForm.tsx: manage localIsFullUrl state, persist to meta on save
- useProviderActions.ts: block switching to isFullUrl or openai_chat
  providers when proxy is not running, show warning toast
- App.tsx: pass isProxyRunning to useProviderActions
- i18n: add fullUrlEnabled/fullUrlDisabled/fullUrlHint and
  proxyRequiredForSwitch translations for zh/en/ja

src-tauri/src/provider.rs

@@ -240,6 +240,9 @@ pub struct ProviderMeta {
     /// - "ANTHROPIC_API_KEY": 少数供应商需要原生 API Key
     #[serde(rename = "apiKeyField", skip_serializing_if = "Option::is_none")]
     pub api_key_field: Option<String>,
+    /// 是否将 base_url 视为完整 API 端点（不拼接 endpoint 路径）
+    #[serde(rename = "isFullUrl", skip_serializing_if = "Option::is_none")]
+    pub is_full_url: Option<bool>,
 }
 
 impl ProviderManager {