refactor: remove LocaleSwitcher component and update locale handling

- Removed LocaleSwitcher from NavUser, SiteHeader, and WorkbenchHeader components.
- Updated tests to reflect the removal of LocaleSwitcher.
- Changed default locale from "en-US" to "zh-CN" in i18n configuration.
- Refactored locale resolution logic to read configured locale without relying on browser language detection.
- Updated AgentDesk SDK to support language configuration from public API.
- Cleaned up unused auth options fetching in the auth API.

docker/agent-desk-lancedb.yaml

@@ -1,3 +1,5 @@
+language: zh-CN
+
 server:
   port: 8083
   cors:

docker/agent-desk-sqlite-lancedb.yaml

@@ -1,3 +1,5 @@
+language: zh-CN
+
 server:
   port: 8083
   cors:

docker/agent-desk.yaml

@@ -1,3 +1,5 @@
+language: zh-CN
+
 server:
   port: 8083
   cors: