feat(KB-051): add duplicate task button in task detail modal

- Add duplicateTask action to useTasks hook for creating task copies

- Add Duplicate button to TaskDetailModal with visual styling

- Wire up duplicateTask action through App.tsx task handlers

- Add comprehensive tests for duplicate button functionality

- Include changeset for the new duplicate task feature

.changeset/add-duplicate-task-button.md

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+---
+"@dustinbyrne/kb": patch
+---
+
+Add duplicate task button to dashboard task detail modal