fix: embed directoryPath in workflow JSON for portable hydration

The directoryPath field was defined in WorkflowFile and checked during
load, but never written during save. Workflows opened from a different
machine or via file picker had no way to find their externalized media
files, so all ref-based nodes appeared empty.

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

src/store/workflowStore.ts

@@ -1956,6 +1956,7 @@ const workflowStoreImpl: StateCreator<WorkflowStore> = (set, get) => ({
         version: 1,
         id: workflowId,
         name: workflowName,
+        directoryPath: saveDirectoryPath,
         nodes: currentNodes,
         edges,
         edgeStyle,