feat(workflow): add workflow engine module (Phase 4)

Implement complete workflow engine with BPMN subset support:

Backend (erp-workflow crate):
- Token-driven execution engine with exclusive/parallel gateway support
- BPMN parser with flow graph validation
- Expression evaluator for conditional branching
- Process definition CRUD with draft/publish lifecycle
- Process instance management (start, suspend, terminate)
- Task service (pending, complete, delegate)
- PostgreSQL advisory locks for concurrent safety
- 5 database tables: process_definitions, process_instances,
  tokens, tasks, process_variables
- 13 API endpoints with RBAC protection
- Timeout checker framework (placeholder)

Frontend:
- Workflow page with 4 tabs (definitions, pending, completed, monitor)
- React Flow visual process designer (@xyflow/react)
- Process viewer with active node highlighting
- 3 API client modules for workflow endpoints
- Sidebar menu integration

apps/web/src/App.tsx

@@ -9,6 +9,7 @@ import Roles from './pages/Roles';
 import Users from './pages/Users';
 import Organizations from './pages/Organizations';
 import Settings from './pages/Settings';
+import Workflow from './pages/Workflow';
 import { useAuthStore } from './stores/auth';
 import { useAppStore } from './stores/app';
 
@@ -46,6 +47,7 @@ export default function App() {
                     <Route path="/users" element={<Users />} />
                     <Route path="/roles" element={<Roles />} />
                     <Route path="/organizations" element={<Organizations />} />
+                    <Route path="/workflow" element={<Workflow />} />
                     <Route path="/settings" element={<Settings />} />
                   </Routes>
                 </MainLayout>