feat(C3): add React Flow workflow editor

frontend/src/pages/WorkflowEditor.tsx: full React Flow editor with
custom nodes (Input/Convert/Render/FFmpeg/Output), config sidepanel,
node palette with drag-drop, new workflow dialog.
frontend/src/api/workflows.ts: workflow CRUD API client.
@xyflow/react added to package.json dependencies.
Route /workflows + sidebar link for admin+pm.

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

frontend/package.json

@@ -10,6 +10,7 @@
   },
   "dependencies": {
     "@react-three/drei": "^9.102.3",
+    "@xyflow/react": "^12.0.0",
     "@react-three/fiber": "^8.16.2",
     "@tanstack/react-query": "^5.28.4",
     "@tanstack/react-table": "^8.14.0",