feat: add flow export/import frontend + backend tests

Frontend:
- ExportFlowModal with JSON/XML format selection + download
- ImportFlowModal with drag-drop file picker + preview step
- rfflowParser for client-side JSON/XML .rfflow parsing
- Export buttons on editor toolbar and library action menus
- Import button on library page next to Create New
- Provenance display for imported flows in editor
- flowTransfer API client + types

Backend:
- Fix regex->pattern deprecation in export endpoint
- 12 integration tests covering export, import, round-trip,
  access control, tag/category creation, version validation

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

frontend/src/components/library/TreeGridView.tsx

@@ -1,5 +1,5 @@
 import { Link } from 'react-router-dom'
-import { Pencil, Globe, Lock, Trash2, GitBranch, FileText, Wrench, Star } from 'lucide-react'
+import { Pencil, Globe, Lock, Trash2, GitBranch, FileText, Wrench, Star, Download } from 'lucide-react'
 import type { TreeListItem } from '@/types'
 import { TagBadges } from '@/components/common/TagBadges'
 import { cn } from '@/lib/utils'
@@ -13,6 +13,7 @@ interface TreeGridViewProps {
   onFolderCreated: (parentId?: string | null) => void
   onDeleteTree: (tree: TreeListItem) => void
   onForkTree?: (treeId: string) => void
+  onExportTree?: (treeId: string) => void
   pinnedTreeIds?: Set<string>
   onTogglePin?: (treeId: string) => void
   pinLoadingTreeIds?: Set<string>
@@ -24,6 +25,7 @@ export function TreeGridView({
   onTagClick,
   onDeleteTree,
   onForkTree,
+  onExportTree,
   pinnedTreeIds,
   onTogglePin,
   pinLoadingTreeIds,
@@ -111,6 +113,20 @@ export function TreeGridView({
               v{tree.version} · {tree.usage_count} uses
             </span>
             <div className="flex items-center gap-2">
+              {onExportTree && (
+                <button
+                  type="button"
+                  onClick={() => onExportTree(tree.id)}
+                  className={cn(
+                    'rounded-md border border-border p-2 text-muted-foreground',
+                    'hover:bg-accent hover:text-foreground'
+                  )}
+                  title="Export flow"
+                  aria-label="Export flow"
+                >
+                  <Download className="h-4 w-4" />
+                </button>
+              )}
               {onForkTree && (
                 <button
                   type="button"