feat: add auto locale switch on first visit based on browser language

[lingxh]

Summary

This PR adds a feature to automatically switch the site language to the user's browser language on their first visit. The behavior is controlled by the NEXT_PUBLIC_AUTO_SWITCH_LOCALE_ON_FIRST_VISIT environment variable and is disabled by default.

Changes

• Add automatic locale detection on first visit using the browser language
• Only trigger the auto-switch on the user's first visit
• Add NEXT_PUBLIC_AUTO_SWITCH_LOCALE_ON_FIRST_VISIT environment variable to control the feature
• Default behavior keeps the feature disabled unless explicitly enabled
• Ensure existing locale preference is not overridden after the first visit

Related Issues

Closes #

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor / code quality improvement
• Chore / dependency update / CI change

Checklist

• I have read the Contributing Guide
• My code follows the project's code style and conventions
• I have run npm run typecheck && npm run lint and there are no errors
• The build passes (npm run build)
• I have tested my changes locally
• I have added or updated documentation if needed
• I have updated translations (locales/) if my changes affect user-facing text
• I have included screenshots or a screen recording for UI changes

Screenshots / Demo

N/A

Notes for Reviewers

• The feature is guarded by the NEXT_PUBLIC_AUTO_SWITCH_LOCALE_ON_FIRST_VISIT environment variable.
• Default value is disabled, so there is no behavior change unless the variable is enabled.
• The auto-switch only happens on the first visit to avoid overriding user-selected language preferences.