feat(KB-651): add changed files diff viewer to dashboard

- Add backend API endpoint /api/tasks/:id/diff for file diffs
- Add useChangedFiles hook for fetching and managing diff state
- Create ChangedFilesModal component with file list and diff viewer
- Integrate modal with TaskCard via files changed badge click
- Add CSS styling for changed files layout and diff display
- Add keyboard navigation (Escape to close)
- Include unit tests for ChangedFilesModal component

packages/dashboard/app/components/WorktreeGroup.tsx

@@ -15,7 +15,7 @@ interface WorktreeGroupProps {
     id: string,
     updates: { title?: string; description?: string; dependencies?: string[] }
   ) => Promise<Task>;
-  onOpenFilesForTask?: (taskId: string) => void;
+  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string) => void;
 }
 
 function WorktreeGroupComponent({