Integrate GraphQL commits filter query with frontend

What does this MR do and why?

Integrates author and message filters into the commit list, enabling users to search commits by author name or commit message content.

Changes:

• Add author and query variables to the commits GraphQL query
• Implement handleFilter method to process filter tokens and update query variables
• Add valueField config to UserToken for flexible value selection
• Fix date grouping to properly handle timezone offsets using toISODateFormat
• Improve error handling to display server error messages when available

References

Screenshots or screen recordings

How to set up and validate locally

1. Navigate to any project's commit list (Repository > Commits)
2. Use the filter bar to search by:
   • Author: Select an author from the dropdown
   • Message: Enter text to search commit messages
   • Free text: Type directly to search messages
3. Verify commits are filtered correctly and clearing filters restores the full list

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #550474 (closed)