feat: advanced search and filtering for work items page

[Flo0806]

Summary

Add search and filtering capabilities to the work items page, similar to what the issues page already has.

Filters

• State: open / closed / merged / draft
• Type: issue / PR / both (issue + linked PR)
• Assigned to me / assigned to specific user
• Author filter
• Labels filter
• Sort: updated / created / comments

Search

• Freetext search in title
• Autocomplete suggestions

Caching

• Cache first page of results server-side
• Invalidate when filters change
• Smooth client-side transitions (no flash on filter change)

Technical Notes

• Extend /api/repository/{owner}/{repo}/work-items with query parameters
• Use GitHub Search API (search/issues) for combined issue+PR search
• Reuse existing filter UI patterns from the issues page