feat(workflows): React Flow canvas editor + list + approvals UI

Adds the admin Workflows surface (top-level nav, gated by the workflows
flag + workflows.view): a list page (enable toggle, delete, new), a
pending-approvals inbox (confirm/deny), and a React Flow (@xyflow/react)
canvas editor — palette to add nodes, drag handle→handle to connect
(branch/gate/loop expose yes-no / confirm-deny / loop-exit handles), a
side-panel JSON config editor, and save (writes a new version). Routes +
sidebar entry + workflows.service. Build + tsc clean.

NOTE: the workflow page strings render via inline English fallbacks; DE
translations for the workflows.* block are still pending native review.

frontend/src/App.tsx

@@ -46,6 +46,9 @@ import { QuoteResponsePage } from './pages/public/QuoteResponsePage';
 import { ContractResponsePage } from './pages/public/ContractResponsePage';
 import { ProjectsListPage } from './pages/admin/projects/ProjectsListPage';
 import { ProjectCockpitPage } from './pages/admin/projects/ProjectCockpitPage';
+import { WorkflowsListPage } from './pages/admin/workflows/WorkflowsListPage';
+import { WorkflowApprovalsPage } from './pages/admin/workflows/WorkflowApprovalsPage';
+import { WorkflowEditorPage } from './pages/admin/workflows/WorkflowEditorPage';
 import { ContractsListPage } from './pages/admin/contracts/ContractsListPage';
 import { ContractEditorPage } from './pages/admin/contracts/ContractEditorPage';
 import { ContractDetailPage } from './pages/admin/contracts/ContractDetailPage';
@@ -314,6 +317,14 @@ function App() {
                       <Route path="customers"     element={<Navigate to="/admin/clients/accounts" replace />} />
                       <Route path="customers/:id" element={<RedirectCustomerDetail />} />
 
+                      {/* Workflows (automation engine) — top-level area gated
+                          by the `workflows` flag. */}
+                      <Route element={<RequireFeature flag="workflows" />}>
+                        <Route path="workflows" element={<WorkflowsListPage />} />
+                        <Route path="workflows/approvals" element={<WorkflowApprovalsPage />} />
+                        <Route path="workflows/:id" element={<WorkflowEditorPage />} />
+                      </Route>
+
                       <Route path="settings" element={<SettingsPage />} />
                       <Route path="system-health" element={<SystemHealthPage />} />
                       <Route path="webhooks/:id/deliveries" element={<WebhookDeliveriesPage />} />