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>

src/shared/contracts/channels.ts

@@ -16,8 +16,13 @@ export const ipcChannels = {
   deletePattern: 'patterns:delete',
   setPatternFavorite: 'patterns:set-favorite',
   saveWorkflow: 'workflows:save',
+  saveWorkflowTemplate: 'workflows:save-template',
   deleteWorkflow: 'workflows:delete',
   listWorkflowReferences: 'workflows:list-references',
+  createWorkflowFromTemplate: 'workflows:create-from-template',
+  exportWorkflow: 'workflows:export',
+  importWorkflow: 'workflows:import',
+  upgradePatternToWorkflow: 'workflows:upgrade-pattern',
   createWorkflowSession: 'workflows:create-session',
   setTheme: 'settings:set-theme',
   setTerminalHeight: 'settings:set-terminal-height',