feat: add confirm dialog and tree delete UI on library page

Adds a reusable ConfirmDialog component and integrates tree deletion
into the TreeLibraryPage with permission-gated delete buttons and
a destructive confirmation dialog.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/common/ConfirmDialog.tsx

@@ -0,0 +1,65 @@
+import { Modal } from './Modal'
+import { cn } from '@/lib/utils'
+
+interface ConfirmDialogProps {
+  isOpen: boolean
+  onClose: () => void
+  onConfirm: () => void
+  title: string
+  message: string
+  confirmLabel?: string
+  confirmVariant?: 'destructive' | 'default'
+  isLoading?: boolean
+}
+
+export function ConfirmDialog({
+  isOpen,
+  onClose,
+  onConfirm,
+  title,
+  message,
+  confirmLabel = 'Delete',
+  confirmVariant = 'destructive',
+  isLoading = false,
+}: ConfirmDialogProps) {
+  return (
+    <Modal
+      isOpen={isOpen}
+      onClose={onClose}
+      title={title}
+      size="sm"
+      footer={
+        <div className="flex justify-end gap-3">
+          <button
+            onClick={onClose}
+            disabled={isLoading}
+            className={cn(
+              'rounded-md border border-border px-4 py-2 text-sm font-medium',
+              'text-card-foreground hover:bg-accent',
+              'disabled:opacity-50 disabled:cursor-not-allowed'
+            )}
+          >
+            Cancel
+          </button>
+          <button
+            onClick={onConfirm}
+            disabled={isLoading}
+            className={cn(
+              'rounded-md px-4 py-2 text-sm font-medium text-white',
+              'disabled:opacity-50 disabled:cursor-not-allowed',
+              confirmVariant === 'destructive'
+                ? 'bg-destructive hover:bg-destructive/90'
+                : 'bg-primary hover:bg-primary/90'
+            )}
+          >
+            {isLoading ? 'Processing...' : confirmLabel}
+          </button>
+        </div>
+      }
+    >
+      <p className="text-sm text-muted-foreground">{message}</p>
+    </Modal>
+  )
+}
+
+export default ConfirmDialog