feat: Add manual reordering support to work items list

What does this MR do and why?

feat: Add manual reordering support to work items list

Add ability to manually reorder work items by drag and drop when sorted by manual/relative position. This brings feature parity with issues list.

  • Add workItemsReorder GraphQL mutation
  • Implement handleReorder method with cache updates
  • Enable manual sort option for non-epic work item types
  • Add isManualOrdering computed property
  • Wire up reorder event handler to IssuableList component
  • Add comprehensive test coverage for reorder scenarios

Manual sorting is disabled for epics to maintain current behavior.

Changelog: added

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Screen_Recording_2025-08-18_at_5.30.59_PM

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Related to #550396 (closed)

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