feat(workflows): add phase 5 backend templates and export support

- add shared workflow template and serialization helpers for YAML, Mermaid, and DOT export
- add pattern-to-workflow upgrade and template application flows in AryxAppService
- persist built-in and custom workflow templates in workspace state and expose new IPC/preload methods
- cover the new backend slice with shared and app-service tests

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/renderer/sessionWorkspace.test.ts

@@ -12,6 +12,7 @@ describe('session workspace helpers', () => {
       projects: [],
       patterns: [],
       workflows: [],
+      workflowTemplates: [],
       settings: createWorkspaceSettings(),
       sessions: [
         {