feat(backend): add create-folder action node

[LukasHirt]

Summary

• Adds a new createFolder action node end to end: pick a Create Folder entry on the canvas, configure a template-rendered path (e.g. /Archive/{{llm.output}}), and the workflow issues a WebDAV MKCOL at that path when it runs.
• Backend: exposes webdavfile.Client's existing private mkcol method as a public CreateFolder(ctx, authHeader, davPath) error, adds it to the executor's FileClient interface, and adds a case "createFolder": arm to runAction that renders the path template against vars and calls CreateFolder. Idempotent (201/405/409 all treated as success), matching the existing internal mkcol usage in Comment.
• Unlike move/copy/rename, this action does not change currentPath — it creates a location rather than relocating the file under consideration. result.Output is set to the rendered path so a subsequent move/copy action can reuse it as its own destination.
• Frontend: adds an action-create-folder entry to NODE_TYPES under the Actions category (icon folder-add), extends ActionType with 'createFolder', and wires the path config field into NodeDetailsPanel.vue following the same v-model/actionParam pattern used by move's destination field.

Test plan

• Backend: cd backend && go test ./... — all packages pass, including new tests in pkg/executor (renders path templates, calls CreateFolder with the rendered path, sets result.Output, and does not mutate currentPath) and new pkg/webdavfile/webdavfile_test.go (exercises CreateFolder's MKCOL request shape and its 201/405/409 idempotency, plus propagation of unexpected statuses).
• go vet ./... and gofmt -l . clean.
• Frontend: npm run test:unit — new tests/unit/nodeTypes.spec.ts asserts action-create-folder exists in NODE_TYPES with the right category/actionType/defaultData and resolves via findNodeTypeForNode.
• npm run check:types and npm run lint clean.

🤖 Generated with Claude Code