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/pages/TreeLibraryPage.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useState, useCallback, useMemo } from 'react'
 import { useNavigate, useSearchParams } from 'react-router-dom'
-import { X, RotateCcw, Play, Sparkles } from 'lucide-react'
+import { X, RotateCcw, Play, Sparkles, FileUp } from 'lucide-react'
 import { treesApi } from '@/api/trees'
 import { categoriesApi } from '@/api/categories'
 import { foldersApi } from '@/api/folders'
@@ -8,6 +8,8 @@ import { sessionsApi } from '@/api/sessions'
 import type { TreeListItem, CategoryListItem, FolderListItem, Session } from '@/types'
 import { FolderEditModal } from '@/components/library/FolderEditModal'
 import { ForkModal } from '@/components/library/ForkModal'
+import { ExportFlowModal } from '@/components/library/ExportFlowModal'
+import { ImportFlowModal } from '@/components/library/ImportFlowModal'
 import { ConfirmDialog } from '@/components/common/ConfirmDialog'
 import { TreeGridView } from '@/components/library/TreeGridView'
 import { TreeListView } from '@/components/library/TreeListView'
@@ -76,6 +78,10 @@ export function TreeLibraryPage() {
   // Fork modal state
   const [forkTarget, setForkTarget] = useState<TreeListItem | null>(null)
 
+  // Import/Export modal state
+  const [showImportModal, setShowImportModal] = useState(false)
+  const [exportTarget, setExportTarget] = useState<TreeListItem | null>(null)
+
   // AI builder state
   const [showAIBuilder, setShowAIBuilder] = useState(false)
   const { aiEnabled } = useCachedQuota()
@@ -250,6 +256,11 @@ export function TreeLibraryPage() {
     if (tree) setForkTarget(tree)
   }
 
+  const handleExportTree = (treeId: string) => {
+    const tree = trees.find((t) => t.id === treeId)
+    if (tree) setExportTarget(tree)
+  }
+
   const hasActiveFilters =
     selectedCategoryId || selectedTags.length > 0 || searchQuery || selectedFolderId
 
@@ -282,6 +293,13 @@ export function TreeLibraryPage() {
                     Flow Assist
                   </button>
                 )}
+                <button
+                  onClick={() => setShowImportModal(true)}
+                  className="flex items-center gap-2 rounded-lg border border-border bg-[rgba(255,255,255,0.04)] px-4 py-2 text-sm font-medium text-foreground hover:border-[rgba(255,255,255,0.12)] transition-colors"
+                >
+                  <FileUp className="h-4 w-4" />
+                  Import
+                </button>
                 <CreateFlowDropdown
                   aiEnabled={aiEnabled}
                   onOpenAIBuilder={() => setShowAIBuilder(true)}
@@ -493,6 +511,7 @@ export function TreeLibraryPage() {
                     setShowDeleteConfirm(true)
                   }}
                   onForkTree={handleForkTree}
+                  onExportTree={handleExportTree}
                   pinnedTreeIds={pinnedTreeIds}
                   onTogglePin={togglePin}
                   pinLoadingTreeIds={pinLoadingTreeIds}
@@ -509,6 +528,7 @@ export function TreeLibraryPage() {
                     setShowDeleteConfirm(true)
                   }}
                   onForkTree={handleForkTree}
+                  onExportTree={handleExportTree}
                   pinnedTreeIds={pinnedTreeIds}
                   onTogglePin={togglePin}
                   pinLoadingTreeIds={pinLoadingTreeIds}
@@ -530,6 +550,7 @@ export function TreeLibraryPage() {
                     )
                   }}
                   onForkTree={handleForkTree}
+                  onExportTree={handleExportTree}
                   pinnedTreeIds={pinnedTreeIds}
                   onTogglePin={togglePin}
                   pinLoadingTreeIds={pinLoadingTreeIds}
@@ -582,6 +603,20 @@ export function TreeLibraryPage() {
           onClose={() => setForkTarget(null)}
         />
       )}
+
+      {exportTarget && (
+        <ExportFlowModal
+          treeId={exportTarget.id}
+          treeName={exportTarget.name}
+          onClose={() => setExportTarget(null)}
+        />
+      )}
+
+      {showImportModal && (
+        <ImportFlowModal
+          onClose={() => { setShowImportModal(false); loadTrees() }}
+        />
+      )}
     </div>
   )
 }