feat(C3): add React Flow workflow editor

frontend/src/pages/WorkflowEditor.tsx: full React Flow editor with
custom nodes (Input/Convert/Render/FFmpeg/Output), config sidepanel,
node palette with drag-drop, new workflow dialog.
frontend/src/api/workflows.ts: workflow CRUD API client.
@xyflow/react added to package.json dependencies.
Route /workflows + sidebar link for admin+pm.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -17,6 +17,7 @@ import NewProductOrderPage from './pages/NewProductOrder'
 import NotificationsPage from './pages/Notifications'
 import PreferencesPage from './pages/Preferences'
 import TenantsPage from './pages/Tenants'
+import WorkflowEditorPage from './pages/WorkflowEditor'
 
 function ProtectedRoute({ children }: { children: React.ReactNode }) {
   const token = useAuthStore((s) => s.token)
@@ -66,6 +67,14 @@ export default function App() {
               </AdminRoute>
             }
           />
+          <Route
+            path="workflows"
+            element={
+              <AdminRoute>
+                <WorkflowEditorPage />
+              </AdminRoute>
+            }
+          />
           <Route path="materials" element={<MaterialsPage />} />
           <Route path="activity" element={<WorkerActivityPage />} />
           <Route path="products" element={<ProductLibraryPage />} />