Add sort variable to epics swimlanes GraphQL query

What does this MR do and why?

This MR updates the frontend swimlanes query to pass TITLE_ASC sort by default, so epics in issue board swimlanes are ordered alphabetically by title instead of by creation date.

Depends on !228567 (merged)

Why was this MR needed?

When using issue board swimlanes grouped by epic, epics appeared in reverse creation order (ID descending). This made it difficult for users to locate specific epics, especially in groups with many of them. The board.epics GraphQL field did not accept a sort argument, so there was no way to control the ordering. The EpicSortEnum and sort_by_attribute already existed in the codebase for other epic endpoints, so this change just wires them through the board epics resolver.

What are the relevant issue numbers?

Fixes #497391 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Ensure the backend MR (!228567 (merged)) is merged or cherry-picked locally
2. Start GDK: gdk start
3. Create or navigate to a group with multiple epics (titles that sort differently than creation order)
4. Create issues and assign them to the epics (or use pre-existing issues)
5. Navigate to the group issue board: http://127.0.0.1:3000/groups/<group>/-/boards?group_by=epic
6. Verify epic swimlanes appear in alphabetical order (A-Z)

Does this MR meet the acceptance criteria?

• Tests updated (feature spec uses explicit epic titles to match new alphabetical order)
• All existing tests passing (19 frontend)
• Follows project style guide
• No breaking changes introduced
• Changelog entry included