feat: add an option to switch to english language

[Pratiyankkumar]

Right now the content of the website is mostly in Chinese , Added an button to switch between Chinese and english language .

Demo Video

[Pratiyankkumar]

PR summary (last two prompts):

1. Error message i18n – Added errors in zh.js / en.js and switched key error messages in Step2, Step3, Step5, and Process.vue to $t() so errors follow the selected language.

2. Backend locale for LLM – Backend uses X-Locale from the frontend to pick the report language. When English is selected, the LLM uses English; when Chinese, it uses Chinese.

3. Report Agent chat language – Report chat API reads locale and passes report_language to ReportAgent. The chat system prompt now includes a language instruction so responses match the user’s language.

4. Localized API error responses – Introduced error_messages.py and updated the report chat API so error messages are returned in the user’s language instead of hardcoded Chinese.

[Pratiyankkumar]

@hzr1937 please review